The interrelation between temperature regimes and fish size in juvenile Atlantic cod (Gadus morhua): effects on growth and feed conversion efficiency

The present paper describes the growth properties of juvenile Atlantic cod (Gadus morhua) reared at 7, 10, 13 and 16 °C, and a group reared under “temperature steps” i.e. with temperature reduced successively from 16 to 13 and 10 °C. Growth rate and feed conversion efficiency of juvenile Atlantic cod were significantly influenced by the interaction of temperature and fish size. Overall growth was highest in the 13 °C and the T-step groups but for different reasons, as the fish at 13 °C had 10% higher overall feeding intake compared to the T-step group, whereas the T-step had 8% higher feeding efficiency. After termination of the laboratory study the fish were reared in sea pens at ambient conditions for 17 months. The groups performed differently when reared at ambient conditions in the sea as the T-step group was 11.6, 11.5, 5.3 and 7.5% larger than 7, 10, 13 and 16 °C, respectively in June 2005. Optimal temperature for growth and feed conversion efficiency decreased with size, indicating an ontogenetic reduction in optimum temperature for growth with increasing size. The results suggest an optimum temperature for growth of juvenile Atlantic cod in the size range 5–50 g dropping from 14.7 °C for 5–10 g juvenile to 12.4 °C for 40–50 g juvenile. Moreover, a broader parabolic regression curve between growth, feed conversion efficiency and temperature as size increases, indicate increased temperature tolerance with size. The study confirms that juvenile cod exhibits ontogenetic variation in temperature optimum, which might partly explain different spatial distribution of juvenile and adult cod in ocean waters. Our study also indicates a physiological mechanism that might be linked to cod migrations as cod may maximize their feeding efficiency by active thermoregulation

Long-term changes of euphausiids in shelf and oceanic habitats southwest, south and southeast of Iceland

Generalized additive models (GAMs) were used to test the hypothesis that changes in physical and biological environmental conditions affected by current climatic warming would negatively impact the euphausiid populations in the North Atlantic. Two zooplankton time series were used, one collected by the Marine Research Institute (MRI) on a transect south of Iceland during spring (1990–2011) and the other by the Continuous Plankton Recorder (CPR) survey (1958–2007) in the oceanic waters south of Iceland covering all months. Due to limitations of the sampling gears used, the results mainly reflect the variations of the early stages of euphausiids. On a spatial scale, results reveal a general decline of euphausiid abundance from the east coast of Greenland to the Faroe Islands. On a temporal scale, euphausiid numbers decreased in most CPR areas from 1958 to 2007. Conversely, an increase was observed in numbers of larvae during spring 1990–2011 for the shelf south of Iceland. Single variable-based GAMs indicated that phytoplankton biomass was generally the main environmental factor regulating euphausiid abundance. Multiple variable-based GAMs showed that phytoplankton biomass was the strongest predictor of euphausiid abundance in the west, whereas in the east temperature appears to be most important. In addition, the onset of the spring bloom also affected the long-term changes in euphausiid abundance. For the oceanic areas, it is concluded that a weakened temporal synchrony between the development of young euphausiids and the phytoplankton bloom influenced by recent climate warming may have led to the observed decrease in euphausiid populations

Ciclo de vida pós-marsupial e crescimento de Metamysidopsis elongata atlantica (Crustacea, Mysidacea, Mysidae) em cultivo de laboratório Pós-marsupial life cicle and growth of Metamysidopsis elongata atlantica (Crustacea, Mysidacea, Mysidae) under laboratory condition

<abstract language="eng">Metamysidopsis elongata atlantica (Bacescu, 1968) was reared in the laboratory for 45 days at 20±1°C and salinity of 30ppt. Growth curves (von Bertalanffy model) were calculated for both sexes and for each sex. The daily rate of carapace growth was significantly different between females and males (F test, p <0.05). Before the sexual maturity (14 days), the growth rate of females was higher than that of males (females, 0.0457 mm day-1; males, 0.0448 mm day-1). After the maturity (15 to 45 days), these rates decreased similarly for both sexes (females, 0.0203 mm day-1; males, 0.0174mm day-1). The average growth rate was 0.0207mm day-1 over the 45 days. Twelve molts were observed in a period of 60 days. The first five molts occurred up to 14 days old (age of the sexual differentiation), with a mean intermolt period of 2.9 days. From the 6th molt it increased to 5.6 days. The results suggest that the use of the carapace length is a good measure to calculate the growth and longevity of the organisms

The trophodynamics of Southern Ocean Electrona (Myctophidae) in the Scotia Sea

The Scotia Sea is one of the most productive regions of the Southern Ocean, but its surface waters are experiencing a rapid increase in temperature, which may be changing the behaviour and distribution of many myctophids and their prey species. Electrona antarctica and Electrona carlsbergi are two of the most abundant myctophids in the region, but their ecology is poorly understood and their response to ongoing environmental change is difficult to determine. This study investigated spatial and temporal patterns in their abundance, population structure and diets using mid-water trawl nets deployed across the Scotia Sea during spring, summer and autumn. E. antarctica was the most numerically abundant species (0.09–0.21 ind. 1,000 m−3), with greatest concentrations occurring in the sea-ice sectors. E. carlsbergi occurred in more northern regions, comprising densities of 0.02–0.11 ind. 1,000 m−3. There was evidence of seasonal variation in depth distribution, size-related sexual dimorphism and size-specific vertical stratification for both species. Latitudinal trends in sex ratio and female body size were apparent for E. antarctica. Its diet varied between regions, seasons and size classes, but overall, Euphausia superba, Metridia spp. and Themisto gaudichaudii were the dominant prey items. E. carlsbergi appeared not to recruit in the Scotia Sea. Its diet was dominated by copepods, particularly Rhincalanus gigas and Metridia spp., but regional, seasonal and ontogenetic variations were evident. This study contributes to our understanding of how mid-water food webs are structured in the Southern Ocean and their sensitivity to ongoing environmental change