Ontogeny matters: Climate variability and effects on fish distribution in the eastern Bering Sea

Analyses of climate effects often ignore differences in life history for individual species. We analyzed a 34‐year time series of eastern Bering Sea fish surveys to evaluate changes in distribution by length and between cold and warm shelf‐wide average water temperatures for 20 species over inhabited depth, temperature, and location. All species showed evidence of ontogenetic migration. Differences in distribution between years with warm and years with cold shelf‐wide water temperatures varied among species and within species at different lengths. For species where shelf‐wide temperature effects were detected, the mid‐sized fish were most active in changing spatial distribution. For aquatic organisms ontogenetic migration occurs because life history stages have different environmental requirements. This study illustrates the need to consider species responses to climate change over different life history stages, and that studies on ecosystem responses should take ontogenetic differences into consideration when assessing impacts.     

Surveillance of selected nutrients,additives and undesirables in commercial Norwegian fish feeds in the years 2000–2010

This article presents data on commercial, Norwegian fish feeds from 2000 to 2010, including elements, additives, some selected vitamins and a range of environmental contaminants. Iodine, selenium and vitamin D concentrations, all declined during the time period; simultaneously, a reduction in marine ingredients in feeds has occurred. Still, the feeds fulfilled known fish nutrient requirements. Arsenic (As) in the feed was fitted with a polynomial regression with peak concentrations in 2004, correlating with fishery landings of blue whiting (Micromesistus poutassou), a reduction species with high content of As. A polynomial regression was also significant for mercury, which peaked in 2005 and was correlated to fish meal (FM) inclusion in the feeds. Residues of the pesticide DDT and its metabolites, chlordane and toxaphene, as well as polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers and sum dioxins and dioxin‐like PCBs, all decreased during this time period. These positive changes in undesirable substances can likely be attributed to the increasing replacement of marine ingredients with plant ingredients, as well as the increased use of South American fish oils rather than North Atlantic ones. On the other hand, cadmium concentrations were twofold higher in South American FMs, and increased in feeds from 2000 to 2010.     

Vertical distribution and migration of fish larvae in the NW Iberian upwelling system during the winter mixing period: implications for cross‐shelf distribution

The vertical distribution and vertical migrations of fish larvae and implications for their cross‐shelf distribution were investigated in the northern limit of the NE Atlantic upwelling region during the late winter mixing period of 2012. The average positive values of the upwelling index for February and March of this year were far from normal, although the average hydrographic conditions during the period of study were of downwelling and the water column was completely mixed. Fish larvae, most in the preflexion stage, were concentrated in the upper layers of the water column and their distribution was depth stratified, both day and night. However, the larval fish community was not structured in the vertical plane and fish larvae did not show significant diel vertical migration (DVM), although five species showed ontogenetic vertical migration. In regions of coastal upwelling and in the absence of DVM, the location of fish larvae in the water column is crucial for their cross‐shelf distribution. Thus, the cross‐shelf distribution of the six most abundant species collected in this study can be explained by the surface onshore flow associated with coastal downwelling, retaining larvae of the coastal spawning species with a relatively shallow distribution in the shelf region and transporting larvae of slope spawning species onto the shelf. The wide vertical distribution shown by larvae of the offshore spawning species could be an adaptation of these species to ensure that some larvae reach the inshore nursery areas.     

Changes in the long‐term distribution of zooplankton in the Humboldt Current Ecosystem off Peru, 1961–2005, and its relationship to regime shifts and environmental factors

We used a dataset assembled by the Peruvian Marine Research Institute (IMARPE) from 1961 to 2005, to examine the influence of seasonal, environmental (temperature and salinity), temporal (day–night and year) and location (north‐south and on‐off shelf) variables on the distribution and biovolume of zooplankton. Classification and regression trees (CART) showed that the zooplankton distribution can be divided into four time regimes: prior to 1974.5, 1974.5–1989.5, 1989.5–1997.5 and 1997.5–2005. These periods differ in overall biovolume but, more importantly, in having different spatial distributions of variables. Offshore‐onshore, seasonal, diel and north‐south patterns are all apparent in the temporal changes observed in zooplankton distribution. In the early period zooplankton biovolume was high throughout the Peruvian Humboldt Current Ecosystem, with enhanced biovolume in all seasons but winter. After 1974.5, biovolume was considerably lower. There was an increased difference between the shelf (lower biovolume) and offshore (higher biovolume), with a significant day–night effect offshore, due to diel migration and possibly diel catchability changes. After 1989.5, seasonal effects were more marked and biovolume appeared to be higher in the winter in the northern region. In other seasons, during the daytime, biovolume was highest farther offshore (greater than 65 km offshore of the 200‐m isobath) and, during the night, higher after 1997.5 than before. We relate these changes in distribution and biovolume to changes in large‐scale current patterns in different regimes and to the possible effect of the fishery in removing nutrients from the system.     

Change in habitat associations and geographic distribution of thorny skate (Amblyraja radiata) in the southern Gulf of St Lawrence: density‐dependent habitat selection or response to environmental change?

We describe changes in the habitat associations and geographic distribution of thorny skate during their feeding season in the southern Gulf of St Lawrence, based on 32 yr of monitoring by a bottom‐trawl survey. In the 1990s, geographic range contracted sharply and distribution shifted into a narrow band of warm deep waters. These changes appeared to reflect altered habitat selection by individual skates, rather than local depletion of the skates that had habitually occupied the vacated areas or a change in the timing of seasonal migrations. Changes in skate distribution coincided with a decline in skate biomass and a cooling of bottom waters. The contraction in the geographic range of skates appeared to be a density‐dependent response, more closely linked to the decline in skate abundance than to the change in environmental conditions. The cause of the shift in habitat associations is less certain. An index of the distribution shift was more strongly correlated with skate biomass than with an index of temperature conditions, and the change in temperature associations is in the direction predicted by density‐dependent bioenergetic considerations. However, the shift into deeper waters is into a depth zone where skate condition is relatively low, contrary to the expectation that fish should be concentrated in optimal habitat at low population size. On the other hand, while the shift into warm deep waters coincided with a cooling of waters at intermediate depths, distribution failed to shift back to the earlier pattern as these waters warmed in the late 1990s.