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Moustahfid, H., Link, J. S., Overholtz, W. J., and Tyrrell, M. C. 2009. The advantage of explicitly incorporating predation mortality into age-structured stock assessment models: an application for Atlantic mackerel. – ICES Journal of Marine Science, 66: 445–454.An age-structured assessment programme (ASAP) that explicitly incorporates predation mortality was applied to Atlantic mackerel (Scomber scombrus) in the Northwest Atlantic. Predatory removals were modelled in the same manner as fishing mortality, with a comparable set of time-series, to produce estimates of predation mortality at age and for each year. Results from the analysis showed that incorporating predation into a mackerel stock assessment model notably altered model outputs. When excluding explicitly modelled rates of predation, the model underestimated the magnitude and uncertainty in spawning-stock biomass (SSB) and recruitment. Further, the rates of predation mortality varied across time and were higher for younger fish. Predation mortality was higher than fishing mortality for fish aged 1 year, approximately equal for 2-year-olds, and lower for older fish (3 years and older). Biological reference points for Atlantic mackerel differed considerably when predation mortality was included. For example, SSBMSY was more than twice as high in the model where predation was incorporated than in the fisheries-only model. Although there are several caveats to the predation model outputs, chief of which is that the estimates are conservative because some mackerel predators were excluded, the results demonstrate the feasibility of executing such an approach with an extant tool. The approach presented here ultimately has the advantage of detecting, and upon detection parsing out, the impact of predators relative to fisheries and has the potential to provide useful information to those interested in small pelagic fish and their associated fisheries. [ABSTRACT FROM PUBLISHER]

The biomass of Atlantic herring Clupea harengus in the Gulf of Maine-Georges Bank (GOMGB) region declined sharply in the mid- to late 1970s, when fishery harvests ranged from 300 to 400 kilometric tons (kt) per year. During this same period, the consumption of Atlantic herring by fish and marine mammal predators was very large. Consumption has greatly exceeded landings from 1985 to the present. Previous research showed that four groups of predators (demersal fishes, marine mammals, large pelagic fishes, and seabirds) together annually consumed over 200 kt of Atlantic herring during the 1970s and over 300 kt during the late 1990s and early 2000s. Our analyses indicate that Atlantic herring predation mortality rates are related to the abundance of both Atlantic herring and predators. Predation mortality rates were relatively low during the 1960s, when Atlantic herring were abundant, but increased in the late 1970s and early 1980s, when Atlantic herring declined. Predation mortality rates declined in the 1990s as Atlantic herring abundance increased. Biological reference points for the Atlantic herring complex (composed of two stocks) are significantly different when predation mortality is included. Several methods were used to assess the available fishery yield given that predator requirements have first been satisfied. In general, all of the approaches indicated that the maximum sustainable yield harvest level for the GOM-GB Atlantic herring complex is lower than that estimated from the single-species assessment in which predation effects are not explicitly accounted for. [ABSTRACT FROM AUTHOR]