Introduction 90Methods 92The production model 92The age structured model 93Accounting for uncertainty and stochasticity 94The early life history 95 AbstractThe relationship between the biomass of reproductively mature individuals (spawning stock) and the resulting offspring added to the population (recruitment), the stock-recruitment relationship, is a fundamental and challenging problem in all of population biology. The steepness of this relationship is commonly defined as the fraction of recruitment from an unfished population obtained when the spawning stock biomass is 20% of its unfished level. Since its introduction about 20 years ago, steepness has become widely used in fishery management, where it is usually treated as a statistical quantity. Here, we investigate the reproductive ecology of steepness, using both unstructured and age-structured models. We show that if one has sufficient information to construct a density-independent population model (maximum per capita productivity and natural mortality for the unstructured case or maximum per capita productivity, natural mortality and schedules of size and maturity at age for the structured model) then one can construct a point estimate for steepness. Thus, steepness cannot be chosen arbitrarily. If one assumes that the survival of recruited individuals fluctuates within populations, it is possible, by considering the early life history, to construct a prior distribution for steepness from this same demographic information. We develop the ideas for both compensatory (Beverton-Holt) and over-compensatory (Ricker) stock-recruitment relationships. We illustrate our ideas with an example concerning bluefin tuna (Thunnus thynnus/ orientalis, Scombridae). We show that assuming that steepness is unity when recruitment is considered to be environmentally driven is not biologically consistent, is inconsistent with a precautionary approach, and leads to the wrong scientific inference (which also applies for assigning steepness any other single value).

The coral reef fish community of Hawaii is composed of hundreds of species, supports a multimillion dollar fishing and tourism industry, and is of great cultural importance to the local population. However, a major stock assessment of Hawaiian coral reef fish populations has not yet been conducted. Here we used the robust indicator variable “average length in the exploited phase of the population ()”, estimated from size composition data from commercial fisheries trip reports and fishery-independent diver surveys, to evaluate exploitation rates for 19 Hawaiian reef fishes. By and large, the average lengths obtained from diver surveys agreed well with those from commercial data. We used the estimated exploitation rates coupled with life history parameters synthesized from the literature to parameterize a numerical population model and generate stock sustainability metrics such as spawning potential ratios (SPR). We found good agreement between predicted average lengths in an unfished population (from our population model) and those observed from diver surveys in the largely unexploited Northwestern Hawaiian Islands. Of 19 exploited reef fish species assessed in the main Hawaiian Islands, 9 had SPRs close to or below the 30% overfishing threshold. In general, longer-lived species such as surgeonfishes, the redlip parrotfish (Scarus rubroviolaceus), and the gray snapper (Aprion virescens) had the lowest SPRs, while short-lived species such as goatfishes and jacks, as well as two invasive species (Lutjanus kasmira and Cephalopholis argus), had SPRs above the 30% threshold.

Su, N-J., Sun, C-L., Punt, A. E., Yeh, S-Z., and DiNardo, G. 2011. Modelling the impacts of environmental variation on the distribution of blue marlin, Makaira nigricans, in the Pacific Ocean. – ICES Journal of Marine Science, 68: 1072–1080. Blue marlin are distributed throughout tropical and temperate waters in the Pacific Ocean. The preference of this species for particular habitats may affect its distribution and vulnerability to being caught. The relationships between the spatial pattern of blue marlin abundance and oceanographic conditions, which may be influenced by climate change, were examined using generalized additive models fitted to catch and effort data from longline fisheries. Distributions of blue marlin density, based on combining the probability of presence and abundance given presence, indicate that there is annual variation in the distribution of blue marlin and that the population apparently moved east during the 1997–1998 El Niño. The interannual variability in blue marlin distribution appears to be associated with El Niño events and is related to shifts in sea surface temperature and the deepening of the thermocline. Models of catch and effort that include oceanographic variables could be used, given predictions from climate models, to explore future changes in distribution, which could then be used to provide management advice related to time-area closures.