Interactions of age-dependent mortality and selectivity functions in age-based stock assessment models

The natural mortality rate (M) of fish varies with size and age, although it is often assumed to be constant in stock assessments. Misspecification of M may bias important assessment quantities. We simulated fishery data, using an age-based population model, and then conducted stock assessments on the simulated data. Results were compared to known values. Misspecification of M had a negligible effect on the estimation of relative stock depletion; however, misspecification of M had a large effect on the estimation of parameters describing the stock recruitment relationship, age-specific selectivity, and catchability. If high M occurs in juvenile and old fish, but is misspecified in the assessment model, virgin biomass and catchability are often poorly estimated. In addition, stock recruitment relationships are often very difficult to estimate, and steepness values are commonly estimated at the upper bound (1.0) and overfishing limits tend to be biased low. Natural mortality can be estimated in assessment models if M is constant across ages or if selectivity is asymptotic. However if M is higher in old fish and selectivity is dome-shaped, M and the selectivity cannot both be adequately estimated because of strong interactions between M and selectivity

California's Pacific bonito resource, its status and management

Pacific bonito, Sarda chiliensis, have become increasingly important to California's sport and commercial fishermen since the early 1960's, but are now showing signs of decline. Recent investigations have revealed much about the bonito's life history and population dynamics. These recent discoveries have been brought together into a document which will serve as a guide to future management actions. Document has 44 pages

Data-limited management reference points to avoid collapse of stocks dependent on learned migration behaviour

AbstractMacCall, A. D. 2012. Data-limited management reference points to avoid collapse of stocks dependent on learned migration behaviour. – ICES Journal of Marine Science, 69: 267–270. Risk of stock collapse associated with the loss of a stock's migrational knowledge may be underestimated by conventional reference points. In this paper, I describe three different approaches: a data-poor precautionary approach is to ensure that individuals spawn at least twice; a data-intermediate approach is to calculate a repeat spawning potential ratio (rSPR), which is analogous to the conventional SPR; and for data-rich cases, age-structured population simulations are recommended.</jats:p

The mortality rate of Engraulis mordax in Southern California

The annual mortality rate for the northern anchovy, Engraulismordax, is estimated to be 66.5% in southern California waters, although the mortality rate increases sharply for older fish. A method for evaluating recruitment regularity and age constancy of mortality is presented. (Document has 25 pages