According to their main life history traits, organisms can be arranged in a continuum from fast (species with small body size, short lifespan and high fecundity) to slow (species with opposite characteristics). Life history determines the responses of organisms to natural and anthropogenic factors, as slow species are expected to be more sensitive than fast species to perturbations. Owing to their contrasting traits, cephalopods and elasmobranchs are typical examples of fast and slow strategies, respectively. We investigated the responses of these two contrasting strategies to fishing exploitation and environmental conditions (temperature, productivity and depth) using generalized additive models. Our results confirmed the foreseen contrasting responses of cephalopods and elasmobranchs to natural (environment) and anthropogenic (harvesting) influences. Even though a priori foreseen, we did expect neither the clear-cut differential responses between groups nor the homogeneous sensitivity to the same factors within the two taxonomic groups. Apart from depth, which affected both groups equally, cephalopods and elasmobranchs were exclusively affected by environmental conditions and fishing exploitation, respectively. Owing to its short, annual cycle, cephalopods do not have overlapping generations and consequently lack the buffering effects conferred by different age classes observed in multi-aged species such as elasmobranchs. We suggest that cephalopods are sensitive to short-term perturbations, such as seasonal environmental changes, because they lack this buffering effect but they are in turn not influenced by continuous, long-term moderate disturbances such as fishing because of its high population growth and turnover. The contrary would apply to elasmobranchs, whose multi-aged population structure would buffer the seasonal environmental effects, but they would display strong responses to uninterrupted harvesting due to its low population resilience. Besides providing empirical evidence to the theoretically predicted contrasting responses of cephalopods and elasmobranchs to disturbances, our results are useful for the sustainable exploitation of these resources.
Abstract. Fish condition indices can be used as biological indicators of the health of individuals and are influenced by numerous external and internal drivers. Like most essential biological traits, they are very sensitive to the spatial heterogeneity occurring in marine ecosystems, which appears at many observational scales. This brings out different ecological processes that can only be revealed either regionally or locally. The scale-dependent spatial variability concerns not only environmental factors, but also anthropogenic activities such as fishing. Understanding these relationships is crucial for improving the spatial management of marine resources, because fish condition considerably affects the sustainability of populations. We explore the influence of density-dependent (intra-specific competition) and densityindependent variables (surface chlorophyll a concentration and fishing impact) on fish condition of three species of harvested gadoids (Merluccius merluccius, Phycis blennoides and Micromesistius poutassou) of the benthopelagic communities off the Balearic Islands (western Mediterranean). This area is characterized by high spatial heterogeneity in the environmental processes and fishing impact. Results show contrasting responses in body condition to the investigated covariates, with species-specific and ontogenetic differences sensitive to the spatial scale of analysis. Some of these responses occur at regional level, while others are more sensitive to local variation. Intra-specific competition shows a very clear effect, which depends on the ontogeny, identifying aggregation areas for recruits and limitation of resources for older individuals. Surface chlorophyll a has always a species-specific regional effect, despite gadoids have a more benthic behavior. Fishing effort displays a heterogeneous impact on fish condition. While spatially contrasting effects are observed at local scale, non-linear regional patterns occur, with positive effects of fishing pressure at intermediate levels. Models analyzing the influence of external drivers in essential fish biological traits such as body condition should consider the spatial variation in responses, especially in highly heterogeneous areas where anthropogenic activities occur. Failing to do so may hide local ecological processes that can be crucial for the persistence of fish populations, which is of paramount relevance for the regional assessment and spatial management of marine exploited resources.
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