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Status of Stocks & Multispecies Assessment Program

SSMA Research Presented at the Alaska Marine Science Symposium

Status of Stocks and Multispecies Assessment (SSMA) Program researchers Libby Logerwell and Steve Barbeaux and Lowell Fritz (National Marine Mammal Laboratory) presented a poster at the 2010 Alaska Marine Science Symposium titled, "Using cooperative acoustic surveys to manage small-scale fisheries in sensitive habitat: The relationships between distribution of Steller sea lions, diet composition, prey biomass distribution and oceanographic properties."

The goal of this research is to investigate whether cooperative biomass surveys are an effective way to manage fisheries at local scales important to predators such as Steller sea lions. The long-term vision is that one or more commercial fishing vessels conducts hydroacoustic surveys in specific areas of Steller sea lion critical habitat prior to commercial fishing beginning in these areas.

Biomass estimates from these surveys would then be used to set a quota for the area surveyed that does not jeopardize the foraging success of Steller sea lions in the area. To design an effective cooperative survey, one needs to know whether the data collected by commercial vessels are of sufficiently high quality and resolution, and sufficiently low variability to assess biomass at local scales. One also needs to know where the fishery would be expected to operate and where Steller sea lions prey upon pollock. Finally, information on the physical oceanographic processes that drive pollock distributions is important for building conceptual models of the interactions between environment, fishing, prey, and predators.

To address these information needs, replicate winter acoustic surveys of pollock were conducted in the central Aleutians from a NOAA research vessel and a commercial vessel equipped with a scientific quality ES60 echosounder. To assess the relative importance of haulouts near pollock spawning areas and near fished areas, data were collected on sea lion distribution and diet. Physical and biological oceanographic data were collected to compare water column properties in areas of high and low pollock biomass.

The study design and data analyses were directed by the following conceptual model: geographic patterns in Steller sea lion haul-out and diet composition are directly related to pollock distribution and abundance and indirectly related to water depth, water column structure, and satellite-derived estimates of chlorophyll. The diet composition of sea lions showed a response to spatial variability in pollock abundance (a functional response), however, the distribution of sea lion abundance did not (a numerical response).

The distribution of sea lions may primarily reflect the distribution of Atka mackerel. Atka mackerel are a dominant prey item for Steller sea lions during all seasons, whereas it appears that pollock (and Pacific cod, along with a suite of demersal species) are important prey primarily in the winter. This suggests that sea lion diets respond to small-scale, short-term distribution of prey. Seasonal diet changes reflect differences in availability due to seasonal differences in spawning and aggregating of various prey species.

To assess the indirect, environmental effects on sea lions, a suite of oceanographic properties were examined as indicators of ocean production. None showed more than a suggestive relationship with the distribution and abundance of pollock biomass, and thus the availability of Steller sea lion prey. However, the processes underlying these oceanographic indicators, such as mixing cold, nutrient rich waters to the surface, may only be relevant during the spring and summer when light levels are sufficient for chlorophyll growth and subsequent food chain productivity to take place.

If oceanographic processes are important for driving the distribution and abundance of pollock in the Aleutian Islands during winter, they were not well represented by the variables examined and a new conceptual model needs to be developed that takes into account whatever other processes are important during this season.

By Libby Logerwell>

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