Fisheries Science Lecture Notes

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MSCI 458 - R. Young, Coastal Carolina University

***Final Exam: The notes for the new material for the exam start HERE

Introduction

What is a fishery?

Miller and Johnson's (1981) definition is "a union of aquatic organisms and humans"
I like my own: "a consumptive harvest of wild aquatic resources"

Consumed for food and for industry (fish meal, animal feed, fertilizer, oil…)
Not just fish -- various shellfish and crustaceans, squid, marine mammals, turtles…

Though related, a fishery is not aquaculture (although hatchery-based fisheries blur the line)

3 basic elements of a fishery:

the resource itself
the aquatic environment
people (fishermen, seafood companies, even the broader scope of everyone who influences the habitat)

Conservation and management

fisheries conservation is the "wise, sustainable use of wild (naturally produced) fisheries resources."
Much of fisheries management is simply conservation, but active management occurs in fisheries such as those supplemented by hatcheries (i.e. managed and enhanced, not just conserved wisely)

Ancient History

2300 BC - government fishing fleets in Sumaria near Persian Gulf (nets, hook and line)
2000 BC - aquaculture in China for carp and other freshwater fish (473 BC, Fan Lee wrote a book on carp aquaculture)
1400 BC - Egyptian elite were fishing for recreation (vs. food)
fishermen prominent in Old and New Testament
Fishery regulations in ancient Rome, feudal England, Mass Bay Colony
Fisheries drove exploration of new world (cod and whaling)

Recent History (from your book and from Greenpeace web page, "Amazing facts about the global fisheries crisis")

Worldwide fisheries harvest seems to have reached a plateau in the last 2 decades at around 85-90 million metric tons (mmt)

Increased over 4 times since 1950 (from 18.5 million tons in 1950) (Greenpeace)
Another 25 mmt or more each year is thrown back as bycatch--most won't survive (Greenpeace)

Since 1970, the world's fishing fleet expanded twice as fast as the world catch (Greenpeace)

overcapacity means pressure to exploit
as a whole, the world fishing fleet has been operating at a multi-billion dollar deficit each year through much of the 1990's (Greenpeace)

Worldwide, about 13 million people make all or a major part of their living from fishing (Greenpeace)

Combined with their immediate families, about 50 million people are directly dependent of fishing (Greenpeace)
Another 150 million people are employed on land, processing fish and servicing fleets as part of their jobs (Greenpeace)
In the U.S., about 300,000 people are employed in coastal-related fisheries alone

The average U.S. citizen eats 16 pounds of fish/shellfish per year

For two thirds of the world's population, fish make up about 40% of their protein consumed (Greenpeace)
Although the amount of fish consumed be people each year has increased with the world catch, the proportion of the catch going to people has decreased (roughly 25-30% goes to fish meal for livestock, etc…) (Greenpeace)

Recreational fishing is not to be overlooked:

In 1990, 36 million people age 16 and over went fishing
The money they spent supported the equivalent of 600,000 full-time jobs in fishing related industries

Fishery trends at the international, national, and regional level

We spent a lot of time reviewing some basic fishery trends at various levels and for various species. Consult your presentation handout and notes.

Aquatic Productivity and Fisheries

The main point: Primary production in the oceans supports many trophic pathways, only some of which are productive for fisheries. Fisheries production (growth and reproduction) at exploitable levels is restricted by physical and biological factors to specific areas.
We tend to view different taxonomic levels with a taxonomic bias.

For example we might think of "phytoplankton" as primary producers and yellowfin tuna as a top predator. "Phytoplankton" is a gross generalization, that includes thousands of species with various and sundry characteristics. We do not generalize flounders, tuna, and anchovies as simply "fish" with the same behavior and ecological role, so why would we lump a diatom and cyanobacteria together into one homogenous category?

Long food chains and significant microbial loops will limit total fisheries production at higher trophic levels.
Trophic transfer efficiency (TE) is believe to average around 10%, but can vary under certain conditions and for different groups.

Up to 20% for some bacterial transfers

Changes in gross growth efficiency (GGE) can alter TE

As the costs of obtaining food go down, the proportion of energy allocated toward growth increases
Locomotion costs represent 2-3 X the standard metabolic rate (SMR) for a typical fish at optimal speed (minimum cost of transport)

So, GGE and therefore TE go up if prey densities are high and locomotion and capture costs are low

Fished species

Important invertebrate fisheries:

Molluscs (clams, oysters, mussels, scallops, cepalopods)
Crustaceans (shrimp, crabs, lobster)
Echinoderms (urchins, sea cucumbers)

A partial list of important vertebrate fisheries:

Engraulidae (anchovy) and Clupeids (Atlantic herring, sardines, menhaden, alewives, shad, sprat) – 27% global catch in 1990
Gadidae – cod, Pollack
hakes
dolphinfish (mahi mahi)
triggerfish
Sciaenidae – drums
Salmonidae – salmon, trout, char
Serranidae – groupers, seabass
Scorpaenidae – rockfish, scorpion fish
Carangidae – jacks
Lutjanidae - snappers
Mugilidae – mullet
Scombridae—tunas and mackerels
Bothidae/Pleronectidae – flounders, halibut
Elasmobranchs – sharks and rays

Abundance

(I think lab 1 covered it pretty well – the obvious point is that we would like to know it exactly, but our estimates usually have a large confidence interval)

Growth

Fish grow their whole lives and rates are highly variable

Indeterminate growth (most fished species)

Variable size increases
Sexually mature before reach max size

Determinate growth

Ex/ crustaceans that undergo molts to grow
Predictable size increases
Sexually mature at max size

Increased size = less predation risk, greater energy storage and size to withstand resource fluctuations, increased fecundity and ability to compete for mates
Changes with life stage

Grow most rapidly early in life (exponential curve, initially)
Growth slows at sexual maturity (energy into gametes, which are lost eventually, unlike body mass: gonadal vs. somatic growth)

Exponential growth curve "breaks" to form a sigmoid curve

Growth curve nearly plateaus when older (larger maintenance cost, more into reproduction)

Variation within species

Fast and slow growing fishes

Fast- may reach sexual maturity earlier, but ultimate size is less (often shooting for critical size by a specific season)
Slow-reach sexual maturity later, but grow more (longer time without emphasizing gonadal growth)

Male/female differences

Often, females mature later ("deferred maturity")

larger size=more eggs

Males of some species have deferred maturity

Typically when males have nests or territories and larger size =more mating events (somewhat mammalian)

Environmental factors on growth

Temperatue and salinity have direct effects on metabolism
Temperature, salinity, rainfall/run-off, etc. effect primary production
Changes in water levels for freshwater fishes
Random encounters of patches -- some fish get big advantage early on simply by chance

Density-dependence

Refers to intra-specific competition
A wide variety of results and opinions

Abundance will affect growth ONLY IF resources are limiting
Limitation fluctuates with fish population size and with carrying capacity (neither stay constant)
Often exploited populations are held at a reduced population size
Density-dependent growth most obvious in years of very high abundance or years of depleted carrying capacity

Modeling growth

Von Bertalanffy growth equation (1938)

Most commonly used, but many exist
Originally developed as a model for human growth processes, and based in part on the balance between anabolism and catabolism
Exponential curve, rather than sigmoid. Thus:

works best for length rather than weight
works best for older segments of the population (when larval stages and early juveniles are a prominent component of the age range, a sigmoid curve is likely).

Formula:

L_t = L_inf (1-e^-k(t-t0)), where

L_t = length at time (actually age) t
L_inf = maximum (asymptotic) length
K = Brody growth coefficient, or the rate at which L_inf is achieved (not a growth rate, but a measure of the rate at which the growth rate declines)
T₀ = theoretical age at which the fish would have zero length, according to the model

Generally, a high K is associated with fast early growth, low age and size t maturity, high reproductive output, short life span, and short max length

Fishing gears and techniques

This is not a complete survey of all the gear and techniques covered in your text, although it has some additional information. You are responsible for all the information in your text.

Harvesting Methods (consult pictures in your text)

Hook and Line Gear

Hooks - single, trebble, circle, barbed vs. barbless
Bait - live bait, cut bait, lures, chumming
Longlines

Also trotlines or setlines
Baited hooks and gangions on mainlines
Usually set on bottom or near surface
Spacing of gangions and materials for lines (ropes, monofilament, wire…) effects capture
Tunas, sharks, billfish, halibut, others…

Active hook and line

Pole-fishing, hand lines, power reels
Drop fishing vs. trolling (not trawling!!)

Active Entrapment Gear

Trawls

Bottom, midwater, and surface trawls
Lead or chain line on bottom (sometimes with rollers or bobbins), float or cork line on top
Some sort of spreader (otter doors vs. beam trawl)
Codend
Effective capture, but often with high bycatch (can partially control with mesh size) and benthic habitat destruction

Dredges

Rigid framed gear
Dig in or drag and disturb the bottom, usually to dislodge or scoop up bivalves (oyster and scallop dredge)

Seines

Long rectangular nets (much longer than deep), again with leadline and floatline
Beach or haul seines

For shallow water, set and hauled by boat or by hand

Purse seines

Large deepwater nets
Set in a circle, then pull purse line to form a bowl, then haul in catch
Good for oceanic schooling fishes (menhaden, tuna…)

Passive Entrapment Gear

Trap nets

Long barriers of netting (leads or wings) extend typically from the shoreline angling toward an entrapment net
Entrapment area is typically a series of connected funnels (some sort of fyke or hoop net)
Use to capture fish that move along the shoreline

Pound nets

A slightly more permanent trap net
Wings and sometimes entrapment area set with semi-permenent stakes

Weirs

Even more permanent version of trap net.
Wings are woven walls of brush, or sometimes actual solid walls of rocks or concrete

Trap nets, pound nets, and weirs are very effective against coastal migratory species -- restricted in many areas
Pots and traps
Usually pots catch invertebrates (lobster, crab) and traps catch fish
Soak time of nets and traps effects performance

Entanglement Gear

Gill nets

Size selective - smaller fish pass through mesh, larger fish can't get head in to be gilled

Trammel nets

Entangle fish, but don't gill them
2 small mesh panels on either side of central large mesh panel
fish pushes smaller mesh through large mesh, catching itself in a pouch
not very size selective, but useful for catch and release with minimal damage

How do you find the fish? - advances in technology

Fish finder sonar
Hydrophones
Spotter planes and helicopters
Satellite images of thermal fronts and plankton blooms

Various Life History and Basic Fish Biology Topics:

Age at Sexual Maturity

Does size matter? -- yes, probably more than age

Age of maturity varies with growth rate (as described above), but often the critical size is the same regardless
Especially for larger, longer-lived species (many smaller fish mature after 1 or 2 years, anyway)
In some cases, size of maturity can decrease as well

Compensatory reaction to fishing:

Growth rate increases and age of maturity decreases (how does this happen?)
ex. Herring growth rate increased 25% and age of maturity decreased by 2 years with a growing fishery (Murphy, 1977)

Age of maturity and mortality

Evolutionary pressures
If likely to die early, better mature early and hope to reproduce at least once

Ex. Scup: 80% juvenile mortality rate, reproduce at age 2

If juvenile mortality risk isn't so bad, but reproductive risk is big, better grow large first to get the most out of that reproductive event

Ex. Atlantic sturgeon (mature at 28 years old), spiny dogfish (mature at 12 years old or more), salmon--be anadromous to grow large

Age of maturity versus lifetime offspring production

Early maturity at a smaller size may offset late maturity at a large size by providing more lifetime spawning seasons
This balance tips the other way the longer larger fish live (high fecundity for large fish)
Mix of successful strategies varies by species and by situation

Ex. - slow-growing Atlantic salmon may mature after 1 year at sea, but fast-growing salmon mature after 2-3 years
Seems backwards, but predation risk for slow-growers in the ocean is high--so reproduce early

Sex reversals and sex ratios

· Dioecious - distinct male and female sexes

· Hermaphroditic - sequential or simultaneous

o Protandrous - male, then female

o Protogynous - female, then male

· Why be a simultaneous hermaphrodite?

o Differential advantage according to sex for various sizes

o Examples

§ if size enhances competition for mates/territoriality, large males can maximize reproductive output

§ if above is untrue, large females can maximize fecundity and therefore reproductive output

§ protogynous is more common, especially among fished species (especially groupers and sea basses (Serranidae), and parrotfishes and wrasses)

· sex ratios skewed in sequential hermaphroditic species, and targeting fisheries can potentially skew them further to point of collapse

o example: Protogynous groupers

§ Hermaphrodites - start as females, large adults become males (opposite of and more common than protandrous hermaphrodites)

§ Sex ratio skewed toward significantly more females

§ Large and spatio-termporally predictable spawning aggregations susceptible to heavy fishing pressure

§ Fisheries target large fish (more males removed

§ If males removed early enough, largest females will change sex

1. Leaves fewer females for that year

2. Leaves smaller females that year (less fecund)

§ If males removed too late, largest females will start to change sex, but won't complete process in time

1. Same problem as above, plus fewer males as well (double whammy)

Reproductive Schedule

How often and how many times do you spawn?
Iteroparity versus semelparity

Strategies for maximum lifetime fecundity

Depends on number of offspring per spawning event and likelihood of survival for future spawnings
Survival and future spawning requires:

Survival from predation during spawning
Recovery from physiological stress of reproduction
Weakened, vulnerable to infection, disease, predation
New gonadal growth
Stress can be high enough to go years in between spawning

Shortnose sturgeon may go 8 years
Atlantic salmon with long freshwater migrations may skip a year, but those with short river migrations spawn every year

If you don't have to worry about this, can put more into one reproductive event

American shad example:

All anadramous, coastal migrators as adults
Southern spawners are semelparous, northern spawners are iteroparous usually

Florida fish mature at younger age and smaller size than New Brunswick fish
Florida fish produce 3 times as many eggs as do first time spawners in New Brunswick (and on average, twice as many eggs in a lifetime)
Florida fish use 70-80% of energy reserves to spawn versus 40-60% for New Brunswick fish
Semelparous fish have a greater average number of offspring per female than iteroparous New Brunswick fish (lifetime totals)

Then why be iteroparous at all?

Greater variability in spawning rivers in north (temperature especially) means some years might have poor survival -- better to spread risk across years

Batch spawners

Typically iteroparous
Release eggs in batches over periods of days, weeks, or months
Ex/ Norwegian cod - 50-250,000 eggs per batch at 3-day intervals over a spawning season of 50 days

Fecundity

Number of eggs produced by a female (per spawning season)
Why not a male? - no correlation between number of sperm and number of offspring
Variability in fecundity
Oviparous, ovoviviparous, and viviparous -- generally much higher fecundity in oviparous species
Fecundity varies with body size and species

Atlantic cod: 200,000 (small females) to 12 million eggs (large females)
Smallmouth bass: 2,000 eggs (small females) to 20,000 eggs (large females)
Ocean sunfish up to 300 million eggs per year

Fecundity increases exponentially with size

F = aL^b, where F is fecundity, L is length, and a and b are species-specific coefficients
Increase in F relative to length is greater (even more exponential) than increase in W (weight) relative to length, so relative fecundity (fecundity per unit mass) increases with size.

Why the big increase with size? - are large adult female cod really 60 times larger than small adult females? No

Apportion energy differently depending on life stage (growth vs. metabolism vs. reproduction)
The longer you live, the less important further survival becomes--take risks for more reproductive success
Example: shad - southern shad (semelparous) is much more fecund than northern shad (iteroparous)

Fecundity vs. egg size vs. larval size

Egg and larval size typically inverse relationship with fecundity

Typical oceanic pattern is many small eggs
Good strategy to exploit patchy food resources

Survival rate and fecundity also typically inverse relationship
Extremes on low fecundity and high survival/egg size/larval size are sharks and many species with benthic eggs

Parental Care

What is parental care?

Includes any combination of post-spawning care of eggs and/or larvae
Includes other reproductive specializations which increase survival, including internal fertilization, direct nourishment of eggs, etc…

Parental care and fecundity generally inversely related
Types:

Broadcast spawning (minimal care)

Ocean spawners - usually pelagic eggs
Freshwater/estuarine spawners - usually demersal eggs

Egg scatterers

Lay demersal eggs on specific substrates, but don't modify them (build nests)
Exs. - sand lance on cont. shelf sand and winter flounder on estuarine sand, walleye over gravel/boulder, Atlantic silversides on intertidal filamentous algae mats (keep moist and hidden)

Shelter spawners

Many species lay demersal eggs in crevices, nooks, etc
Some lay eggs on live inverts (sea ravens and sculpins on sponges, etc.)

Nest builders

Many in gravel (good aeration and protection) - lamprey, salmon and trout (redds), fallfish (see picture in book)
Depressions in sediment - sunfishes, freshwater bass
On vegetative material - damselfish and garibaldi (algal matt), sticklebacks construct tubular nests of fragments of vegetation

Guarding

Many nest builders then guard (damselfish)
Why benefit males more? (attract more mates?) - your book takes this view only
Or is it females that benefit? (less energy for parental care--more for egg production?)
There are also alternative "cheating" strategies for subordinate males

Brooding

Internal (ovoviviparous and viviparous elasmobranchs, guppies, etc.- carry eggs inside until hatch/born)
External - carry fertilized eggs or larvae outside of female's reproductive tract (pouches, mouths, gills, etc..)

Recruitment

Recruitment is the addition of new individuals through reproduction and growth

Used differently depending on context

Recruitment to spawning population
Recruitment to the fishery (the harvestable population)
Recruitment to a reef or substrate
Larval recruitment of demersal species (i.e. "settlement")
Recruitment to "metamorphosis" stage

Recruitment varies greatly from year to year

Routinely varies by a factor of 20 in many species
Abundance of any given year-class or cohort within the fish population is not predictable
Common to have population dominated by one or a few year-classes

Fisheries have often failed to match removal rates with recruitment, often for complex reasons

Lake sturgeon in Great Lakes

High fecundity, so high recruitment, right? No

Late maturity (20 years)
Non-annual spawning (can go years between events)

Does specific protection against recruitment overfishing help? (i.e. protect them completely until they have spawned)

Striped bass - huge success with this strategy

With failing stocks in the 1980's, protected 1 good year class for most of the decade until they could spawn (commercial moratorium, increasing size and number limits for recreational fishing up to 36 inches)
Year class spawned, had a successful year class of its own, and stocks came roaring back

BUT, it doesn't always work that way

A big spawning event during a bad year won't help
The striped bass strategy could have failed in conditions prevented another good year class for several years

Stock Recruitment Models

Relate size of spawning stock to expected rate of recruitment to the spawning stock (i.e. how well does the size of the spawning stock predict recruitment?)
3 primary models (see figures in book)

Ricker Model - assumes density-dependent decrease in recruitment as spawning stock size increases

Recruitment peaks at some mid-level abundance, then declines due to increased competition, cannibalism, etc.)
In reality, stocks can only get so far along the post-peak decline before it becomes self-regulating
You could see this kind of response, though, for stocks which significantly overshoot their carrying capacity and have a strong compensatory reaction (a decline or oscillation back below K)

Beverton and Holt Model - assumes continual but slowing increase (toward a max asymptote) in recruitment as spawning stock size increases

What you might expect for a stock that gradually approaches it's carrying capacity

Shepherd - yet another version with slight differences
Typically, none of the models is all that effective in predicting recruitment from spawning stock size

Correlation is often poor (although seems to do OK for a few species)
Models also make very different predictions from the same data set -- somewhat troubling

Some other problems:

Can't expect a perfect correlation between spawners and recruits -- if so, sustainable fishing would be impossible (removal of adults would lead to ever-declining populations)
If density-dependent intraspecific controls dominated the process, we would see and increase in larval survival/growth as total recruitment declines
The number of spawners may not always be a good proxy for the number of eggs produced -- particularly in an exploited population where declining fish sizes leads to declining fecundity.
The fish stock must be accurately defined: if you count recruits from one stock but spawners from several stocks, you wouldn't expect a correlation.

Recruitment success is often controlled more by environmental and biological factors during early life history, described below

Egg and Larval Mortality

Mortality rates can vary greatly from year-to-year (primary cause of recruitment variability)

Success of a given year-class is often determined by the end of the third month

If recruitment is high at that stage, the year-class will be strong

Estimate a daily egg and larval death rate of 2-10% for typical broadcast spawners

90-99% rainbow smelt eggs die before hatching
70-85% American shad larvae die 4-9 days after hatching
daily mortality rate of newly hatched capelin larvae may exceed 50%
Winter flounder and striped bass larvae have 1.2 and 1.5% average daily death rate

Contrast with 90% survival of brown trout eggs in redds to hatching (3 month incubation period)
Further contrast with 5-10% annual mortality rates for juveniles and adults of typical large-bodied species (generally higher for small-bodied species)
Causes of mortality:

Environmental causes ("Member-vagrant hypothesis": oceanographic features retain larvae in favorable habitats)

(effect growth rates, settlement location, food availability)

Water temperature
Ocean currents or river flow
Wind/waves (waves needed to free capelin larvae from sand/pebble substrate
River discharge (affects salinity, nutrient input, plankton productivity)
Water levels (effect amount of quality shallow spawning habitat - walleye)

Biological causes

Predation ("Bigger is Better Hypothesis")

Predation by whom?

Copepods, chaetognaths, jellies
Small fishes (sometimes turn the table: high bluegill dnesities cause high mortality of largemouth bass eggs and larvae)
cannibalism

size-dependent predation (outgrow predators)
complex interactions (predation may restrict useable space and foraging time, leading to increased competition, slower growth, and longer vulnerability to predation

Starvation ("Match-mismatch Hypothesis": larval hatching time and location must match in space and time with food production)

"critical period" after yolk sac absorption (typically lose weight first)

must eat during this period
controversy about its importance

plankton patch hypothesis

must exploit high density patches to be successful - may include a big random component
planktonic larvae are likely to be concentrated by currents into areas of patches for the same reasons

All of the above causes play a role, and sometimes they conflict with one another

Example: rapid growth may help to avoid predation, but it also requires a higher metabolism and greater starvation risk, or it could be at the expense of energy for locomotion, thereby increasing predation risk, etc…
Overwintering issues

Temperate fish usually timed to be reaching a critical size for survival by winter
If anything slows growth (environmental or biological reasons), can have significant year-class mortality

Fisheries Compensation and Depensation

· Sustainable fishing depends on a compensatory response

o As the number of spawners declines (through fishing removal), the number of recruits per spawner increases, as does the production rates

o This enables replacement of the removed portion of the population at a sustainable level

· At critically low abundance levels, some species may show an opposite trend: depensation

o This leads rapids to a population crash and possibly no recovery

§ There are no known examples of fishing a marine species to extinction, although "commercial extinction" has been seen many times

o Depensation could occur, for example, due to changes in the ability to find suitable mates, or successfully fertilize, or if aggregations at low population densities intensify predation risk, etc.

Movements

· Station-keeping, foraging, territorial movements: significant part of daily activities, but not covered in depth here

· Ideal free distribution movements

o Fish populations (or any animal) will move around, but will distribute proportional to the availability of resources

o Therefore, as exploited populations decline, they will continue to be abundant in areas of prime resources, making them vulnerable to fishing until severely depleted and making their abundance seem to be greater than it is (example, northern cod during long collapse of 1980's and 90's)

· Migrations

o Somewhat fuzzy definition, although clearly a repetitive and predictable pattern of movement between specific destinations

o Important to ecosystem for nutrient transport and food availability to predators (examples: salmon, grunts, menhaden)

o Adult migrations are fairly obvious, (examples: salmon runs, cod spawning "highways,"…) but larvae migrate as well

§ Selective tidal transport brings many shelf-spawned larvae into coastal estuaries along the east coast.

§ Timing and location of spawning takes advantage of physical processes

§ Examples:

· prevailing seasonal winds and longshore transport for red drum along Texas Coast

· fronts across mid-Atlantic shelf for bluefish larvae

· Areas of upwelling, Gulf Stream eddies and meanders, Gulf Stream ring collisions that retain larvae on shelf…

o Diadromous migrations

§ Anadromy (hatch in fresh water, adults in sea water, return to spawn in fresh water)

§ Catadromy (opposite)

§ Anadromy found in temperate and cool waters, where productive ocean waters offer adults the ability to grow very large. In tropical latitudes, the freshwaters are more productive, which promotes catadromy.

Yield and Single-species stock assessment

Surplus Production Models

These are basically extensions of the Logistic equation (Verhulst-Pearl)

where,

o N = population size in number of individuals

o T = time, usually in years

o K = carrying capacity (or max population)

o R = intrinsic rate of population growth (or the potential growth rate when unrestricted by K)

o (often, the (K-N)/K term is written as 1-N/K, but I think that obscures the obvious role of that term in the equation, in that the closer N gets to K, the slower

o the rate of population increase becomes)

After integration and algebraic manipulation, an alternative form of the logistic equation is:

where N₀ = initial population at time t

Even a simple equation like this can lead to chaotic results (see web links to chaos sites from the discussion questions page)

Chaos is when seemingly random and unpredictable (non-linear) patterns are derived from predictable first principles
Slight changes in the initial parameters lead to disproportionately large differences in results with iterative models

The figure below demonstrates several similarly shaped curves that are slightly different in meaning.
In figure below, the derived equation above for N_(t) (above) determines the position along curve "a", while the former equation (for dN/dt) describes the curve "d" below it. Often, however, you will see the later equation given along with curve "a" (because it is the more intuitive equation combined with the more intuitive figure).

Curves "b" and "e" above are basically the same model as the logistic equation for population, but they use biomass instead of number of individuals (so dB/dt instead of dN/dt)

dB/dt can be thought of as the "instantaneous rate of surplus production" (similar to dN/dt "instantaneous rate of population growth")
the shaded areas in figure "b" represent the surplus production or potential yield in one year's time, depending on the stock size.

The potential yield is largest if populations are fished down to the a level in which their dB/dt is maximized

"growth overfishing" - removes too many small fish and limits the stock below the range for maximum yield (mean size of fish in harvest is too small)
"recruitment overfishing" - removes too many adult fish, thereby reducing the reproductive contribution such that the stock is limited below the range for maximum yield
Catch-22:

Prolonged fishing pressure genetically selects for decreased age and size of maturity, and therefore decreased fecundity
Leads to smaller and less reproductively productive stocks in the long run -- bummer!

if fishing effort correlates with removal rates, fishing effort is a measure of population size (it is inversely related to biomass, or yield)

therefore, you can graph yield vs. effort just as you graphed yield vs. rate of surplus production (curve "f" above)

assumes catchability remains constant as fishing effort changes
the max catch is at top of curve, but CPUE is declining as it approaches the max catch

curve "c" above is also a sigmoid curve, but in this case it is the age vs. weight growth curve for an individual fish (lots of similar curves with different meanings)

Yield-per-Recruit (YPR) Models
Addresses the contribution of each cohort to the stock, and addresses the contribution of survival and growth between consecutive ages

Determines maximum yield by finding the balance between natural mortality and growth

Yield-per-recruit models allow managers to play with the impact on yield of:

Age of 1st capture
Fishing mortality rates

So, YPR models can predict the effect of different harvest rates on yield from fishes that are recruited into the stock , but they don't address potential contribution of stock to reproductive output (unlike stock-recruitment models)

Primary advantage: info about stock-recruitment relationships is not needed (often fairly undependable anyway, as discussed earlier)
Management is based on how to best utilize the fish once they have recruited to the fishery, not on how the fishery will effect recruitment
Addresses effect of growth overfishing, but not recruitment overfishing

But recruitment overfishing does occur, so how do you address it? See SSB/R and EPR models below.

A basic YPR model (Van den Avyle, 1993):

where:

o F = instantaneous rate of fishing mortality

o Nt = number of fishes of age t

o Wt = average weight of fishes of age t

o For ages between tc (age of first capture or when 1st vulnerable to fishery) and tl (maximum age in fishery)

Notice that Y = F*N*W for each age category, or in other words the yield (or surplus biomass) = rate of fishing mortality x stock biomass

Add it up separately for each age class and you've got the total change in yield over time

For a YPR model, you need estimates for:

Natural mortality rate (i.e. without fishing) and population size

Used to determine the number of individuals in each age class (Nt)

Growth relationships, including age-length and length-weight relationships

Used to determine the average weight for each age class (Wt)

Information on the "selectivity" of the fishing gear

Used to determine the max and min sizes, and therefore ages, captured in the fishery: (tc and tl).

YPR models assume that natural mortality and growth parameters are independent of the level of fishing mortality, F (i.e. they are constant)

Some extensions/modifications of yield-per-recruit models do allow examination of recruitment overfishing:

Spawning stock biomass per recruit models (SSB/R)

Examine effect of age of first capture and rate of fishing mortality (as before) on reproductive potential of population
Spawning stock biomass = number of fishes at each age x proportion that are mature at each age x average weight at each age (just adding a proportion that are mature factor to the original formula)
SSB/R will decrease as fishing mortality (F) increases

When is it too much (i.e. when does recruitment overfishing occur?)

Often we don’t know the stock-recruitment relationship, or it is unpredictable
Managers have some general "rules of thumb" based more on experience with overfished stocks than with any scientific explanation

Fishing mortality should not reduce the SSB to less than 30% of it’s unexploited level (below that, tend to get rapid declines and recruitment overfishing)
Fishing mortality rates should not exceed natural mortality rates
Often these 2 cut-offs are pretty close to one another

As before, low levels of fishing mortality applied at young ages can have same effect as high mortality applied at older age

There are also eggs per recruit models (EPR)

Examines effort of age of first capture and rate of fishing mortality (as before) on maximum lifetime fecundity of the average female
Must add a factor to original formula that determines proportion that are female and average fecundity for each age or weight class
Increasing fishing mortality has greatest effect on species with

Late age of maturity, or
Large difference between age of 1st capture and age of maturity

Cohort Models

Calculate declining abundance of year classes as they age through a fishery
Ex. Virtual Population Analysis

Uses estimates of natural mortality, fishing mortality for oldest age taken in most recent year, and annual catch statistics for each age
This is more predictive for each cohort than yield-per-recruit models
Develops estimates of annual fishing mortality for each age, and estimates total stock abundance

All models have limitations and assumptions!

Beverton and Holt yield-per-recruit model: assumes fishing mortality is equal for all ages vulnerable to capture
Surplus production models: assumes catchability does not change with fishing effort (ignores changes in vessels, gear, methods, or clustering of fish)
Virtual Population Analysis: instantaneous rate of fishing mortality for oldest age group is not easy to nail down
Catch data can be confusing for mixed commercial and recreational fisheries
Etc.

Multispecies assessment and ecosystem modeling

Single species models often don't reflect accurate practices and fishery realities

· Technical interactions - fishing mortality on more that one species (target or bycatch)

o Ex/ groundfish trawls in Georges Bank keep cod, haddock, winter and yellowtail flounder, Pollock, hake, etc…..

o Ex/ tropical reef fisheries are almost always multi-species

o Size-specific gear targets different species with different impacts

· Biological interactions - interspecific competition and predation

o Increase in one species may lead to decrease in another….

Multispecies Surplus Production Models

· Compare total catch (yield) vs. effort (or F)

· Total MSY is not the same as MSY for different species

o Will over or underfish individual stocks

· Tropical studies sometimes add it up for all sites (islands, etc.), and then look at where each falls out on the curve (fig 8.3)

Multispecies YPR Models

· Compare total yield per total recruit

· Suffers from same problems of "lumping" that multispecies surplus production models do, but allow predictive manipulations of typical YPR models

Multispecies VPA (MSVPA)

· If you remember, single species VPA starts when cohort abundance goes to zero, then adds back in the removals due to fishing and natural mortality the previous year to get the total abundance the previous year, and on up the line.

· MSVPA breaks M (natural mortality) into 2 components:

o Predation to do predators that are included in model

o Natural mortality due to all additional sources

· Example: Fig 8.5

· Can also model whether predator/prey dynamics follow:

o Type II functional response (predation proportional to prey abundance)

o Type III functional response (predation not proportional to prey abundance due to prey switching at low prey abundance -- search images, etc.)

o Gets much more realistic: ex fig. 8.8, where natural mortality of cod drops dramatically as they grow due to decreases in predation from other fish.

· Limitation: required enormous detail on predation patters by cohort for all species in community

o Improved estimates of M from MSVPA can be re-inserted into single species VPA models to improve them

Ecosystem models

· Ecopath with Ecosim, Stella, FishSim, etc….

Fishers: Socioeconomics and human ecology

Who is responsible for fisheries resources?

Fisheries resources are "public property" (unless entirely on privately owned habitats)
Government generally assumes responsibility for their management, paid for by:

your tax money, and
special fees and taxes for user groups (fishing licenses, boat taxes, etc…)

3 categories of fisheries resource users: subsistence, recreational, and commercial harvesters

Subsistence users

Contrary to popular opinion, subsistence does not mean primitive (although it can be)
Subsistence users are common worldwide, but not in the US, right?

Many people in US with low disposable income obtain much of their food from hunting and/or fishing—technically, that’s subsistence

Recreational users (of fishing, not drugs!)

Popularity has increased greatly in this century with increases in leisure time and disposable income

In 1991, more than 36 million anglers fished a total of 511 million days in the US
85% of this effort was in fresh water

No such thing as a typical angler

Some are happy with any catch, others just to be on the water, others with lots of fish, others with only trophy fish
Surveys of more frequent anglers indicate they place greater evidence on the really good catches (trophy fish, rare fish, etc.) and on experiencing time outdoors with family or friends
Less frequent anglers are mostly interested in catching some fish

The growth of recreational fishing has slowed in the US in the 1980s and 90s

Less fishing licenses means less money for management
Compensating by raising license fees may further decrease numbers – catch 22
But, do you want everyone out fishing?

Difficult to manage – a dispersed and unorganized group (although some segments are highly organized and effective)

Commercial users

Who are commercial fishermen?

Hard working, dedicated individuals
The money is often moderate to poor, so many do it because they love it (outdoors, independent, proud)
Strong component of community tradition and family business
Often a strong ethnic and cultural component, by port
Often, each port has its own set of unwritten rules and territorial traditions
Most are independent (not working for a large parent company, except some large fisheries, like tuna fleet)

Who impacts fish stocks more, commercial or recreational fisheries?