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Fisheries ecologists traditionally aimed at disentangling climate and fishing effects from the population dynamics of exploited marine fish stocks. However, recent studies have shown that internal characteristics and external forcing (climate and exploitation) have interactive rather than additive effects. Thought most of these studies explored how demographic truncation induced by exploitation affected the response of recruitment to climate, identifying a general pattern revealed to be difficult as interactions are often case-specific. Here we compared five exploited stocks of European hake Merluccius merluccius from the Atlantic Ocean and Mediterranean Sea to investigate how the interaction between internal characteristics and external forces affect the variability of the population growth rate and their consequences on recruitment. Our results show that demographic truncation induces a novel population scenario in which the growth rate is maximized when the reproductive stock is younger and less diverse. This scenario is shaped by the climate variability and the fishing pattern. The population growth rate becomes more dependent on the maturation schedule and less on the survival rates. The consequences for the recruitment dynamics are twofold; the effect of density-dependent regulatory processes decreases while the effect of the density-independent drivers increases. Our study shows that the interaction between internal characteristics and external forces changes across geographic locations according to 1) the importance of demographic truncation, 2) the influence of the climate on the regional hydrography and 3) the spatiotemporal heterogeneity of the physical environment to which fish life history is adapted.

The purpose of this article is to assess the effectiveness of the collaboration between stakeholders and scientists in the construction of a bio-economic model to simulate management strategies for the fisheries in Iberian Atlantic waters. For 3 years, different stakeholders were involved in a model development study, participating in meetings, surveys and workshops. Participatory modelling involved the definition of objectives and priorities of stakeholders, a qualitative evaluation and validation of the model for use by decision-makers, and an iterative process with the fishing sector to interpret results and introduce new scenarios for numerical simulation. The results showed that the objectives of the participating stakeholders differed. Incorporating objectives into the design of the model and prioritizing them was a challenging task. We showed that the parameterization of the model and the analysis of the scenarios results could be improved by the fishers’ input: e.g. ray and skate stocks were explicitly included in the model; and the behaviour of fleet dynamics proved much more complex than assumed in any traditional modelling approach. Overall, this study demonstrated that stakeholder engagement through dialogue and many interactions was beneficial for both, scientists and the fishing industry. The researchers obtained a final refined model and the fishing industry benefited from participating in a process, which enables them to influence decisions that may affect them directly (to shape) whereas non-participatory processes lead to management strategies being imposed on stakeholders (to be shaped).

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Age determination of redfish is difficult. In this paper, the ages of Sebastes mentella on the Flemish Cap are validated by following year classes from 1991 to 2000. The criteria used for S. mentella are consistent and coherent. The growth of different year classes is described and compared, and density-dependence is demonstrated to influence the growth rate of the strong 1990 year class, growth of that year class being the slowest of those followed. The slow rate of growth prevented that year class from maturing at the anticipated age. Growth is also compared between sexes, of S. mentella, S. marinus, and S. fasciatus, revealing that females grow faster than males. Finally, growth rate is compared among species. S. marinus grows fastest and S. mentella slowest, although the influence of density-dependent growth in S. mentella needs to be taken into consideration.

We use a bootstrap simulation framework to evaluate the relative importance of different sources of random and systematic error when estimating diet or food consumption of cetaceans, using a data set on harbour porpoise diet in Scottish (UK) waters from 1992–2003 (N=180) as a model. We also evaluate the consequences of applying explicit weightings to individual samples and/or sub-sets (‘strata’) of samples. In terms of the precision of estimates of diet composition, sampling error was the most important source of error, to the extent that overall 95% confidence limits changed only very slightly when sub-sampling error and regression errors were taken into account. On the other hand, for estimates of total food consumption by the porpoise population in Scottish waters, uncertainties about population size and energetic requirements were more important than uncertainty about diet composition. In relation to the accuracy of estimates of diet composition, the study also highlighted the importance of selecting regressions appropriate to prey in the study area (as opposed to ones constructed for the same prey species in another area) and demonstrated that applying equal weighting to individual samples or sample strata can substantially alter the resulting picture of diet. Therefore, the rationale for applying such weightings needs to be carefully considered.

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