Presented by Christy Morgan, Chris Sullivan & Jessica Davisson

Factors in Cetacean Distribution

• Physical oceanographic features:
• topographic relief,
• temperature,
• salinity,
• thermo cline topography
• Abundance of prey
• prey abundance due to zooplankton abundance.
• Fish make up the largest portion of the diet of cetaceans (Gaskin, 1976)
• Cetacean feeding grounds tend to be concentrated on the edges of upwelling zones
• feeding ground conditions are most favorable where there is a maximum vertical stability in the water column
• zooplankton and phytoplankton accumulate in these areas
• fish tend to concentrate their feeding along the areas with the heaviest zooplankton densities
• prey fish abundance explains the variation in the cetacean community

Factors in Zooplankton Distribution

• Thermal gradients
• Structural heterogeneity of the continental shelf
• Persistence of hydrographic fronts at the continental shelf margins
• Zooplankton biomass tends to be highest in the slope water of the Gulf Stream, intermediate (but more variable) at the north wall of the Gulf Stream, and lowest in the central Stream (Allison, 1986)
• There is a seasonal trend of the zooplankton biomass, with a maximum in late spring, and a minimum in autumn
• This seasonal trend could be due to seasonal cycles of the sea surface temperature, which affects zooplankton transport
• The habitat use by cetaceans may be related to sea surface temperature (Reilly, 1994)

Hypothesis

• The structure of a zooplankton community can be used as an indicator in the habitat description of odontocetes
• Zooplankton abundance and diversity are useful in understanding odontocete habitat utilization.
• This study examines the relationships between zooplankton abundance and diversity and odontocete distributions off of the northeast coast of the United States
• There is an indirect relationship between odontocete distributions and zooplankton community structure
• There is a trophic relationship between the prey of the odontocetes and the zooplankton community

MATERIALS & METHODS

• The zooplankton were sampled using both the CTD recordings and the Bongo tow method
• the samples were taken randomly from stations at the end of the transects
• The zooplankton samples were divided into subgroups of about 500 organisms and analyzed for taxa and abundance
• Copepods were analyzed to species level
• The abundance data and metered tow volume were used to calculate zooplankton density per meter cubed
• copepod and total organism abundances were log transformed (COPELOG) to compensate for potential curvi-linearity

Bongo Tow Method

• Uses a double "Bongo" frame to carry two nets
• One of the nets has a mesh of 303 micrometers, the other with 505 micrometers.
• The frame was towed at a speed, which could maintain a 30-degree incline from the vertical of the tow wire, and was lowered to a depth of 200m.

Shannon Diversity Index

• Represents the ratio of abundance to richness
• H’= - p_i ln (p_i)
• H’= diversity index
• p_i = the decimal fraction of individuals belonging to the i^th species

Jacquard Index

• Calculates the species abundance equitability (J)
• J = H’/ H’_max
• H’_max= the value of H’computed with the same number of species, but with equal p_i values
• The hydrographic data taken with the CTD was summarized into 4 variables:
• maximum temperature above 200m depth (TEMPMAX),
• minimum temperature above 200m depth (TEMPMIN),
• the difference between maximum and minimum temperatures (TEMPRANG), and
• water masses classified according to thermal structure (THERCLAS)
• thoroughly mixed over the entire water column sampled =0
• thermally stratified with warm water overlying cooler water =+1
• thermally stratified with warm water underlying cooler water =-1
• All odontocete sightings from the vessel were recorded
• the sightings were represented as total number of animals within 20 km of the stations used along the transects, in a survey block of 20 km
• Species sighting rates were calculated when any odontocete species was found in 50% of the survey blocks used
• species included:
• Common dolphin (Delphinus delphis), striped dolphin (Stenella coeruleoalba), Atlantic white-sided dolphin (Lagenorhynchus acutus), pilot whales (Globicephala spp.), sperm whales (Physeter macrocephalus), and beaked whales (Mesoplodon spp.)
• Specific species were analyzed for abundance and group size relationships with hydrographic conditions and zooplankton variables
• The species had to be spotted within 20 km of 16 or more stations
• Pearson’s product-moment correlation coefficient was used for correlations with environmental parameters

DISCUSSION

• In the study, the sperm whale, common dolphin and striped dolphin sighting rates correlated positively with bottom depth. This indicates that during the survey these species were inhabitants of deeper or off shelf water.
• White sided dolphin sighting rates correlated negatively with depth. Suggesting that these species was occupying a shallower niche during the study.
• There were high rates of correlation between physical and biological parameters. The study suggests that cetacean distribution was affected indirectly through bathymetry. There is a strong association between physical characteristics and community structure. (Ashjian and Wishner, 1992). Bathymetry was responsible for effects on current flow, upwelling of nutrients and the biological production that occurred because of these interactions. Bathymetry has also been sighted as an important selective pressure in the evolution of Cetacians. (Berta & Sumich, 1999).
• Common dolphin and striped dolphin sighting rates were negatively correlated with copepod abundances and positively correlated with copepod diversity. Areas that were high in diversity and low in abundance were in warmer areas of higher productivity.

Allison, S.K and K. F. Wishner. 1986. Spatial and temporal patterns of zooplankton biomass across the Gulf Stream. Mar. Ecol. Prog. Ser., 31: 233-244.

Asjian, C.J. and K.F Wishner. 1993. Temporal persistence of copepod species groups in the Gulf Stream. Deep-Sea Res. 40: 483-516.

Berta, A. and J. L Sumich. 1999. Marine Mammals Evolutionary Biology. Academic Press, San Diego.

Fiedler, P.C. and S.B Reilly. 1994. Inter annual variability of dolphin habitats in the Eastern tropical pacific. Effects on abundances estimated from tuna vessel sightings.

Gaskin, D. E. 1976. The evolution, zoogeography and ecology of cetaceans. Oceanogr. Mar. Biol. Ann. Rev., 14: 247-346.

Reilly. S. B., and P.C. Fielder. 1994. Inter annual variability of dolphin habitats in the eastern tropical Pacific. 1: Research vessel surveys. 1986 – 1990. Fish. Bull., 92: 434- 450.

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