Jean-Marc Fromentin

Ecosystem oceanography for global change in fisheries

Overexploitation and climate change are increasingly causing unanticipated changes in marine ecosystems, such as higher variability in fish recruitment and shifts in species dominance. An ecosystem-based approach to fisheries attempts to address these effects by integrating populations, food webs and fish habitats at different scales. Ecosystem models represent indispensable tools to achieve this objective. However, a balanced research strategy is needed to avoid overly complex models. Ecosystem oceanography represents such a balanced strategy that relates ecosystem components and their interactions to climate change and exploitation. It aims at developing realistic and robust models at different levels of organisation and addressing specific questions in a global change context while systematically exploring the ever-increasing amount of biological and environmental data.

Global climate change and phenotypic variation among red deer cohorts

The variability of two fitness–related phenotypic traits (body weight and a mandibular skeletal ratio) was analysed among cohorts and age–classes of red deer in Norway. Phenotypic variation among cohorts was pronounced for calves, yearlings and reproductively mature adults. Fluctuations in cohort–specific mean body weights and skeletal ratios of adults correlated with global climatic variation in winter conditions influenced by the North Atlantic Oscillation while cohorts were in utero. Red deer born following warm winters were smaller than those born after cold winters, and this inter–cohort variability persisted into adulthood. Phenotypic variation among cohorts of red deer influenced by climate change may pose consequences for fitness of cohorts since body size and condition contribute to reproductive success and survival in male and female red deer. In particular, the recent trend of increasingly warm winters in northern Europe and Scandinavia may lead to reduced body size and fecundity of red deer, and perhaps other ungulates, in those areas.

Fishing effects and life history traits: a case study comparing tropical versus temperate tunas

This study aims to test whether exploitation affects tunas and tuna-like species displaying contrasting life history traits similarly. We first collected information on life history of 10 commercial Atlantic species and then compared this information using multivariate analysis. On one hand, tropical tunas are characterised by small to medium size, rapid growth, early age-at-maturity, long spawning duration and short life span. These species, therefore, display a rapid turnover, characteristic of r-selected species. On the other hand, temperate tunas display differing life history traits, i.e., large size, slow growth, late age-at-maturity, short spawning duration and long life span. The turnover of these species is slow and present characteristics similar to ‘K-selected’ species (with a conservative strategy adapted to a colder and more variable environment). We, then, selected the two tuna species displaying the most contrasting life histories, i.e., skipjack (SKJ) and bluefin tuna (BFT), and investigated their respective responses to various levels of exploitation, using simulation modelling. If fishing activity starts at age 1 (a situation which is close to the actual exploitation pattern), differences in life history traits make the BFT population much more fragile to exploitation and less productive than SKJ. However, if the fisheries only target adults, both SKJ and BFT populations are able to sustain high F. Spawning stocks and yields of BFT also display conspicuous long-term fluctuations, resulting from the combination of year-to-year variations in the recruitment and a long life span. This variability makes it difficult to detect overfishing or depletion risks in the BFT population. Because of its short life span, SKJ does not display such long-term variations in its SSB. Our simulations also showed that current management measures based on a minimum size limit are much more critical for BFT than SKJ. This difference stresses the importance of taking account of differences in life history traits into management measures.

EFFECTS OF DENSITY-DEPENDENT AND STOCHASTIC PROCESSES ON THE REGULATION OF COD POPULATIONS

We analyzed 136 time series (covering from 44 to 73 yr) of juvenile cod to estimate the level of direct and delayed density-dependent mortality (DDM) of 11 popu- lations from the Norwegian Skagerrak coast. The parameters were estimated using a mod- eling approach that explicitly incorporates observation errors, so that we could quantify the density-independent (stochastic) variation in the survival of juvenile cod. Moderate to strong levels of DDM (direct or delayed) were estimated in eight of the 11 populations. Variability in the 0-group (corrected for observation errors) appeared to be large for most of the populations. Substantial stochastic variability in postsettlement survival was also detected in some areas, indicating that stochastic factors are not only important for egg and larval stages, as stated by the match-mismatch hypothesis, but also for juveniles. We show that the variability in these coastal populations is not only regulated as a function of the strength of DDM processes, but also as an interaction between DDM processes and stochastic factors. We finally postulate that local and regional differences in the strengths of the density-dependent and stochastic processes are related to differences in the quantity and quality of the bottom flora coverage, which govern both food availability and shelter for juveniles.

A NEW TEST FOR DENSITY‐DEPENDENT SURVIVAL: THE CASE OF COASTAL COD POPULATIONS

A new test based on the generalized additive model is proposed to investigate density-dependent mortality in the juvenile cohorts of cod. Density dependence implies that the function linking the count of a cohort in one year to the count in the succeeding year is convex. The method estimates (without functional assumptions) the function linking the two counts and provides a level of significance for any convexity. We investigate the power and bias of the new test on the basis of simulated data. The power compares well with a test of unit slope in a log-log plot (although it is usually somewhat lower). However, in contrast to the latter method, the test for convexity is much more resistant to measurement error. We applied the model to long-term survey data from two areas of the Norwegian Skagerrak coast. In both cases, the variance is intermediate between the Gamma (variance proportional to the squared mean) and the Poisson (variance proportional to the mean) distributions. A negative binomial (with k - 3.5) describes the variance well. The variance is interpreted as resulting from sampling errors, spatial heterogeneity, and environmental stochasticity. Incorporating this error structure, the optimal models linking the two main juvenile stages are, for each area, nonlinear and significantly convex (P < 0.05). The full models are highly significant (P < 0.001), and the examination of the residuals does not reveal any remaining structure. We conclude that the survival of juvenile cod along the Norwegian Skagerrak coast is density dependent, probably because of cannibalism, com- petition for habitat, and food limitation. The functional form of density-dependence in the per capita survival rate is estimated to be approximately log-linear.

NATURAL MORTALITY AND FISHING MORTALITY IN A COASTAL COD POPULATION: A RELEASE–RECAPTURE EXPERIMENT

As with most teleosts, the life cycle of the cod (Gadus morhua) is characterized by high productivity of eggs leading to high concentration of larvae. These stages are known to be associated with high mortality rates. Mortality of older stages (juveniles and adults) has been relatively poorly studied. We use capture–mark–recapture (CMR) methodology to estimate time and age variation of natural mortality and fishing mortality in a coastal cod population within the Norwegian Skagerrak coast. A total of 36 728 reared 6-mo-old, and 2415 wild-caught mostly 18-mo-old, individually tagged cod were released in four consecutive years, leading to 4155 recaptures by fishermen. Tag loss and mortality due to tagging appeared negligible for reared young fish, but up to 60% of the wild-caught tagged fish disappeared just after release. Tag return by fishermen was ∼50–60%. Natural mortality was found to be high in the 6–12 mo range, but subsequently it decreased rapidly and was no longer age-dependent. Fishing mortality ...

Spatial structure and distribution of small pelagic fish in the northwestern Mediterranean Sea.

Understanding the ecological and anthropogenic drivers of population dynamics requires detailed studies on habitat selection and spatial distribution. Although small pelagic fish aggregate in large shoals and usually exhibit important spatial structure, their dynamics in time and space remain unpredictable and challenging. In the Gulf of Lions (north-western Mediterranean), sardine and anchovy biomasses have declined over the past 5 years causing an important fishery crisis while sprat abundance rose. Applying geostatistical tools on scientific acoustic surveys conducted in the Gulf of Lions, we investigated anchovy, sardine and sprat spatial distributions and structures over 10 years. Our results show that sardines and sprats were more coastal than anchovies. The spatial structure of the three species was fairly stable over time according to variogram outputs, while year-to-year variations in kriged maps highlighted substantial changes in their location. Support for the McCalls basin hypothesis (covariation of both population density and presence area with biomass) was found only in sprats, the most variable of the three species. An innovative method to investigate species collocation at different scales revealed that globally the three species strongly overlap. Although species often co-occurred in terms of presence/absence, their biomass density differed at local scale, suggesting potential interspecific avoidance or different sensitivity to local environmental characteristics. Persistent favourable areas were finally detected, but their environmental characteristics remain to be determined.

Spatio-temporal dynamics of species richness in coastal fish communities

Determining patterns of change in species richness and the processes underlying the dynamics of biodiversity are of key interest within the field of ecology, but few studies have investigated the dynamics of vertebrate communities at a decadal temporal scale. Here, we report findings on the spatio–temporal variability in the richness and composition of fish communities along the Norwegian Skagerrak coast having been surveyed for more than half a century. Using statistical models incorporating non–detection and associated sampling variance, we estimate local species richness and changes in species composition allowing us to compute temporal variability in species richness. We tested whether temporal variation could be related to distance to the open sea and to local levels of pollution. Clear differences in mean species richness and temporal variability are observed between fjords that were and were not exposed to the effects of pollution. Altogether this indicates that the fjord is an appropriate scale for studying changes in coastal fish communities in space and time. The year–to–year rates of local extinction and turnover were found to be smaller than spatial differences in community composition. At the regional level, exposure to the open sea plays a homogenizing role, possibly due to coastal currents and advection.

Spatio-temporal patterns of key exploited marine species in the Northwestern Mediterranean Sea.

This study analyzes the temporal variability/stability of the spatial distributions of key exploited species in the Gulf of Lions (Northwestern Mediterranean Sea). To do so, we analyzed data from the MEDITS bottom-trawl scientific surveys from 1994 to 2010 at 66 fixed stations and selected 12 key exploited species. We proposed a geostatistical approach to handle zero-inflated and non-stationary distributions and to test for the temporal stability of the spatial structures. Empirical Orthogonal Functions and other descriptors were then applied to investigate the temporal persistence and the characteristics of the spatial patterns. The spatial structure of the distribution (i.e. the pattern of spatial autocorrelation) of the 12 key species studied remained highly stable over the time period sampled. The spatial distributions of all species obtained through kriging also appeared to be stable over time, while each species displayed a specific spatial distribution. Furthermore, adults were generally more densely concentrated than juveniles and occupied areas included in the distribution of juveniles. Despite the strong persistence of spatial distributions, we also observed that the area occupied by each species was correlated to its abundance: the more abundant the species, the larger the occupation area. Such a result tends to support MacCalls basin theory, according to which density-dependence responses would drive the expansion of those 12 key species in the Gulf of Lions. Further analyses showed that these species never saturated their habitats, suggesting that they are below their carrying capacity; an assumption in agreement with the overexploitation of several of these species. Finally, the stability of their spatial distributions over time and their potential ability to diffuse outside their main habitats give support to Marine Protected Areas as a potential pertinent management tool.

Effects of Stochasticity in Early Life History on Steepness and Population Growth Rate Estimates: An Illustration on Atlantic Bluefin Tuna

The intrinsic population growth rate (r) of the surplus production function used in the biomass dynamic model and the steepness (h) of the stock-recruitment relationship used in age-structured population dynamics models are two key parameters in fish stock assessment. There is generally insufficient information in the data to estimate these parameters that thus have to be constrained. We developed methods to directly estimate the probability distributions of r and h for the Atlantic bluefin tuna (Thunnus thynnus, Scombridae), using all available biological and ecological information. We examined the existing literature to define appropriate probability distributions of key life history parameters associated with intrinsic growth rate and steepness, paying particular attention to the natural mortality for early life history stages. The estimated probability distribution of the population intrinsic growth rate was weakly informative, with an estimated mean r = 0.77 (±0.53) and an interquartile range of (0.34, 1.12). The estimated distribution of h was more informative, but also strongly asymmetric with an estimated mean h = 0.89 (±0.20) and a median of 0.99. We note that these two key demographic parameters strongly depend on the distribution of early life history mortality rate (M0), which is known to exhibit high year-to-year variations. This variability results in a widely spread distribution of M0 that affects the distribution of the intrinsic population growth rate and further makes the spawning stock biomass an inadequate proxy to predict recruitment levels.