2016 Multi-Species Stock Assessment For Walleye Pollock, Pacific Cod, And Arrowtooth Flounder In The Eastern Bering Sea

MSCAA models may increase forecast accuracy, may be used to evaluate propagating effects of observation and process error on biomass estimates, and can quantify climate and trophic interactions on species productivity. As such MSCAA models can address long recognized limitations of prevailing single species management, notably non-stationarity in mortality and maximum sustainable yield (MSY), and may help reduce risk of overharvest. Because multispecies biological references points (MBRPs) from MSCAA model are conditioned on the abundance of other species in the model, they may also have utility in setting harvest limits for multi-species fleets, evaluating population dynamics in marine reserves or non-fishing areas, and quantifying trade-offs that emerge among fisheries that impact multiple species in a food web.

Depending on their structure, MSCAA models can be used to evaluate climate-and fisheries-driven changes to trophodynamic processes, recruitment, and species abundance. MSCAA models differ somewhat among systems and species, but most use abundance and diet data to estimate fishing mortality, recruitment, stock size, and predation mortality simultaneously for multiple species in a statistical framework. Similar to age structured single species stock assessment models widely used to set harvest limits, MSCAA models are based on a population dynamics model, the parameters of which are estimated using survey and fishery data and maximum likelihood methods. Unlike most single-species models, MSCAA models additionally separate natural mortality into residual and annually varying predation mortality, and model the latter as a series of predator-prey functional responses. Thus, natural mortality rates for each species in MSCAA models depend on the abundance of predators in a given year and vary annually with changes in recruitment and harvest of each species in the model.

MSCAA models have specific utility in quantifying direct and indirect effects of fisheries harvest on species abundance and size distributions, which is important for EBFM and trade-off analyses of various management strategies. Rapidly shifting climate conditions are also of growing concern in fisheries management as changes in physical processes are known to influence individual growth, survival, and reproductive success of fish and shellfish. Climate-driven changes in water temperature can directly impact metabolic costs, prey consumption, and somatic or gonadal tissue growth, with attendant indirect effects on survival, production, and sustainable harvest rates. Temperature-dependent predation, foraging, metabolic, and growth rates are common in more complex spatially-explicit food web or whole of ecosystem models such as GADGET, Atlantis, and FEAST. Temperature functions for growth and predation can also be incorporated into MSCAA models, allowing this class of models to be used to evaluate interacting climate, trophodynamic, and fishery influences on recommended fishing mortality rates.

Numerous studies point to the importance of using multi-species models for EBFM. Multi-species production models produced different estimates of abundances and harvest rates than single species models for Northeast US marine ecosystems, and MSY of commercial groundfish stocks estimated from aggregated production models are different than the sum of MSY estimates from single-species assessments. Multi-species models have been used to demonstrate long-term increases in yield of Icelandic stocks of Atlantic cod and reductions in capelin and Northern shrimp catch associated with short-term decreases in cod harvest. Kaplan demonstrated the disproportionately large ecosystem impacts of applying the same Fx (e.g., Fx, or the harvest rate that reduces spawning stock biomass to x% of unfished spawning stock biomass; harvest control rule approach to forage fish as is used for groundfish in the northeast Pacific, and trophodynamics in a southern Benguela ecosystem resulted in higher carrying capacity for small pelagic species under fishing (versus no-fishing) scenarios.