Methodological elements for optimising the spatial monitoring design to support regional benthic ecosystem assessments.

Benthic habitat condition assessments are a requirement under various environmental directives. The Marine Strategy Framework Directive (MSFD), for example, challenges member states in a European sea region to perform comparable assessments of good environmental status and improve coherence of their monitoring programmes by 2020. Currently, North Sea countries operate independent monitoring programmes using nationally defined assessment areas. Lack of an agreed OSPAR or EU scale monitoring method and programme has been identified as a priority science need. This paper proposes a method for the development of a coherent and efficient spatial sampling design for benthic habitats on regional level and gives advice on optimal monitoring effort to get more accurate assessments. We use ecologically relevant assessment areas (strata) across national borders and test spatial sample allocation methods. Furthermore, we investigate the number of samples needed in each stratum to reduce the variance for estimating mean number of taxa and abundance. The stratification needs to take into account the spatial heterogeneity of the entire ecosystem. The total sample effort is optimal when sample allocation takes into account the size and benthic variability within those strata. Change point analysis helps to find a balance between sampling effort and precision of the benthic parameter estimate. A joint sampling design for the North Sea could be generated by combining current efforts, and where needed adapting existing national programmes. This serves a coordinated, region-wide, benthic condition status assessment and strengthens regional cooperation to fulfil multiple monitoring tasks, with a scientifically underpinned common approach.

Contrasting patterns of genetic and phenotypic differentiation in two invasive salmonids in the southern hemisphere.

Invasion success may be expected to increase with residence time (i.e., time since first introduction) and secondary releases (i.e., those that follow the original introduction), but this has rarely been tested in natural fish populations. We compared genetic and phenotypic divergence in rainbow trout and brown trout in Chile and the Falkland Islands to test the prediction that adaptive divergence, measured as P ST/F ST, would increase with residence time and secondary releases. We also explored whether interspecific competition between invaders could drive phenotypic divergence. Residence time had no significant effect on genetic diversity, phenotypic divergence, effective population size, or signatures of expansion of invasive trout. In contrast, secondary releases had a major effect on trout invasions, and rainbow trout populations mostly affected by aquaculture escapees showed significant divergence from less affected populations. Coexistence with brown trout had a positive effect on phenotypic divergence of rainbow trout. Our results highlight an important role of secondary releases in shaping fish invasions, but do not support the contention that older invaders are more differentiated than younger ones. They also suggest that exotic trout may not have yet developed local adaptations in these recently invaded habitats, at least with respect to growth-related traits.

Mixed-effects modelling of scale growth profiles predicts the occurrence of early and late fish migrants.

Fish growth is commonly used as a proxy for fitness but this is only valid if individual growth variation can be interpreted in relation to conspecifics' performance. Unfortunately, assessing individual variation in growth rates is problematic under natural conditions because subjects typically need to be marked, repeated measurements of body size are difficult to obtain in the field, and recaptures may be limited to a few time events which will generally vary among individuals. The analysis of consecutive growth rings (circuli) found on scales and other hard structures offers an alternative to mark and recapture for examining individual growth variation in fish and other aquatic vertebrates where growth rings can be visualized, but accounting for autocorrelations and seasonal growth stanzas has proved challenging. Here we show how mixed-effects modelling of scale growth increments (inter-circuli spacing) can be used to reconstruct the growth trajectories of sea trout (Salmo trutta) and correctly classify 89% of individuals into early or late seaward migrants (smolts). Early migrants grew faster than late migrants during their first year of life in freshwater in two natural populations, suggesting that migration into the sea was triggered by ontogenetic (intrinsic) drivers, rather than by competition with conspecifics. Our study highlights the profound effects that early growth can have on age at migration of a paradigmatic fish migrant and illustrates how the analysis of inter-circuli spacing can be used to reconstruct the detailed growth of individuals when these cannot be marked or are only caught once.

And the last shall be first: heterochrony and compensatory marine growth in sea trout (Salmo trutta).

Early juvenile growth is a good indicator of growth later in life in many species because larger than average juveniles tend to have a competitive advantage. However, for migratory species the relationship between juvenile and adult growth remains obscure. We used scale analysis to reconstruct growth trajectories of migratory sea trout (Salmo trutta) from six neighbouring populations, and compared the size individuals attained in freshwater (before migration) with their subsequent growth at sea (after migration). We also calculated the coefficient of variation (CV) to examine how much body size varied across populations and life stages. Specifically, we tested the hypothesis that the CV on body size would differ between freshwater and marine environment, perhaps reflecting different trade-offs during ontogeny. Neighbouring sea trout populations differed significantly in time spent at sea and in age-adjusted size of returning adults, but not on size of seaward migration, which was surprisingly uniform and may be indicative of strong selection pressures. The CV on body size decreased significantly over time and was highest during the first 8 months of life (when juvenile mortality is highest) and lowest during the marine phase. Size attained in freshwater was negatively related to growth during the first marine growing season, suggesting the existence of compensatory growth, whereby individuals that grow poorly in freshwater are able to catch up later at sea. Analysis of 61 datasets indicates that negative or no associations between pre- and post-migratory growth are common amongst migratory salmonids. We suggest that despite a widespread selective advantage of large body size in freshwater, freshwater growth is a poor predictor of final body size amongst migratory fish because selection may favour growth heterochrony during transitions to a novel environment, and marine compensatory growth may negate any initial size advantage acquired in freshwater.