A statistical model for estimation of fish density including correlation in size, space, time and between species from research survey data

Trawl survey data with high spatial and seasonal coverage were analysed using a variant of the Log Gaussian Cox Process (LGCP) statistical model to estimate unbiased relative fish densities. The model estimates correlations between observations according to time, space, and fish size and includes zero observations and over-dispersion. The model utilises the fact the correlation between numbers of fish caught increases when the distance in space and time between the fish decreases, and the correlation between size groups in a haul increases when the difference in size decreases. Here the model is extended in two ways. Instead of assuming a natural scale size correlation, the model is further developed to allow for a transformed length scale. Furthermore, in the present application, the spatial- and size-dependent correlation between species was included. For cod (Gadus morhua) and whiting (Merlangius merlangus), a common structured size correlation was fitted, and a separable structure between the time and space-size correlation was found for each species, whereas more complex structures were required to describe the correlation between species (and space-size). The within-species time correlation is strong, whereas the correlations between the species are weaker over time but strong within the year

Could Seals Prevent Cod Recovery in the Baltic Sea?

Fish populations are increasingly affected by multiple human and natural impacts including exploitation, eutrophication, habitat alteration and climate change. As a result many collapsed populations may have to recover in ecosystems whose structure and functioning differ from those in which they were formerly productive and supported sustainable fisheries. Here we investigate how a cod (Gadus morhua) population in the Baltic Sea whose biomass was reduced due to a combination of high exploitation and deteriorating environmental conditions might recover and develop in the 21st century in an ecosystem that likely will change due to both the already started recovery of a cod predator, the grey seal Halichoerus grypus, and projected climate impacts. Simulation modelling, assuming increased seal predation, fishing levels consistent with management plan targets and stable salinity, shows that the cod population could reach high levels well above the long-term average. Scenarios with similar seal and fishing levels but with 15% lower salinity suggest that the Baltic will still be able to support a cod population which can sustain a fishery, but biomass and yields will be lower. At present knowledge of cod and seal interactions, seal predation was found to have much lower impact on cod recovery, compared to the effects of exploitation and salinity. These results suggest that dual management objectives (recovery of both seal and cod populations) are realistic but success in achieving these goals will also depend on how climate change affects cod recruitment

Localisation of nursery areas based on comparative analyses of the horizontal and vertical distribution patterns of juvenile Baltic cod (Gadus morhua)

Knowledge of the spatial distribution of juvenile cod is essential for obtaining precise recruitment data to conduct sustainable management of the eastern and western Baltic cod stocks. In this study, the horizontal and vertical distribution and density patterns of settled juvenile 0- and 1-group Baltic cod are determined, and their nursery areas are localised according to the environmental factors affecting them. Comparative statistical analyses of biological, hydrographic and hydroacoustic data are carried out based on standard ICES demersal trawl surveys and special integrated trawl and acoustic research surveys. Horizontal distribution maps for the 2001-2010 cohorts of juvenile cod are further generated by applying a statistical log-Gaussian Cox process model to the standard trawl survey data. The analyses indicate size-dependent horizontal and distinct vertical and diurnal distribution patterns related to the seabed topography, water layer depth, and the presence of hydrographic frontal zones (pycnoclines) as well as intraspecific patterns in relation to the presence of adult cod. The extent of the nursery areas also depends on the cod year class strength. Juvenile cod (≥3 cm) are present in all areas of the central Baltic Sea (CBS), showing broad dispersal. However, their highest density in the Baltic Basins is found at localities with a 40-70 m bottom depth in waters with oxygen concentrations above 2 ml O₂.l⁻¹ and temperatures above 5°C. The smallest juveniles are also found in deep sea localities down to a 100 m depth and at oxygen concentrations between 2-4 ml O₂.l⁻¹. The vertical, diurnally stratified and repeated trawling and hydroacoustic target strength-depth distributions obtained from the special surveys show juvenile cod concentrations in frontal zone water layers (pycnocline). However, the analyses indicate that in the CBS, juvenile cod of all sizes do not appear to aggregate in dense schooling patterns, which differs from what has been reported from the North Sea

Interannual Variation In Gonadal Maturation Of Cod In The Gotland Basin Of The Baltic Sea: Influence Of Environment And Fish Condition

No abstracts are to be cited without prior reference to the author.Analysis of the long-term biological data sets (1967-1990) on diet, liver weight and maturation of cod revealed the general trends in accumulation of energy in the liver and gonadal maturation dynamics. The study confirms relationship between energetic condition of cod and food supply. The liver condition index of cod and abundance of clupeids in cod food during the autumn/winter and spring was positively correlated. The sex- and maturation-specific differences in cod diet, liver weight, and utilization of energy for gonadal growth and spawning were defined. The ripening females on the spawning grounds were hardly feeding on clupeids. Liver condition index of females continued to increased and reached the maximum at the final maturation stage. The feeding intensity and liver condition index of maturing males decreased earlier than of females at the final gonad ripening stage. Cod of both sexes at spawning condition nearly ceased feeding. The liver condition index declined. In 1967-1977, at favourable oxygen/temperature conditions on the spawning grounds, the gonad development trends prior to the spawning in January determined the gonad growth during the main spawning season in March-April. During the 1980s the spawning population of cod was influenced by poor hydrographic conditions - low oxygen saturation and temperature. The changes in distribution and age composition of the spawning population of cod caused a decline in proportion of males on the spawning grounds. The diverging courses of proportion of maturing cod in prespawning and spawning seasons indicated that hydrographic conditions likely were the cause of the gonad development delay and spawning disturbance during the stagnation period of 1980s

Interannual variation in gonadal maturation of cod in the Gotland Basin of the Baltic Sea: influence of environment and fish condition

Analysis of the long-term biological data sets (1967-1990) on diet, liver weight and maturation of cod revealed the general trends in accumulation of energy in the liver and gonadal maturation dynamics. The study confirms relationship between energetic condition of cod and food supply. The liver condition index of cod and abundance of clupeids in cod food during the autumn/winter and spring was positively correlated. The sex- and maturation-specific differences in cod diet, liver weight, and utilization of energy for gonadal growth and spawning were defined. The ripening females on the spawning grounds were hardly feeding on clupeids. Liver condition index of females continued to increased and reached the maximum at the final maturation stage. The feeding intensity and liver condition index of maturing males decreased earlier than of females at the final gonad ripening stage. Cod of both sexes at spawning condition nearly ceased feeding. The liver condition index declined. In 1967-1977, at favourable oxygen/temperature conditions on the spawning grounds, the gonad development trends prior to the spawning in January determined the gonad growth during the main spawning season in March-April.   During the 1980s the spawning population of cod was influenced by poor hydrographic conditions - low oxygen saturation and temperature. The changes in distribution and age composition of the spawning population of cod caused a decline in proportion of males on the spawning grounds. The diverging courses of proportion of maturing cod in prespawning and spawning seasons indicated that hydrographic conditions likely were the cause of the gonad development delay and spawning disturbance during the stagnation period of 1980s. </p

Size Structure And Feeding Ecology Of Fish Communities In The Surf Zone Of The Eastern Baltic

No abstracts are to be cited without prior reference to the author.Benthic and pelagic fishes of the nearshore area of the Eastern Baltic were sampled by beach seine in 1998–2002. During the summer-autumn seasons the shallow waters were inhabited by marine juveniles and small-sized fish species. Juvenile diadromous or freshwater fishes stay nearshore for a shorter time period and were recognized as migrants. Trophic interrelations among more abundant and frequently occurring juveniles in the early years of life prior to maturity were studied. Multivariate analysis of the diet similarities between fish species resulted in determination of six trophic guilds: copepod feeders, bathyporeia feeders, mysidae feeders, mysid – copepod feeders, mysid – bathyporeia feeders and piscivores. Food composition of different size classes of juvenile flounder (Platichthys flesus) showed high variety in prey taxa. Ontogenetic shifts from copepods and small amphipods to macrofauna in flounder and from mysids to fish food in turbot (Scophthalmus maximus) occurred as fish grew. The majority of fish diets (7 out of 9 species) included mysids, and turbot was the main consumer of this prey. Besides, juveniles of turbot and perch (Perca fluviatilis) as well as adult greater sandeel (Hyperoplus lanceolatus) were found to be also piscivores. However, for several species, mysids occurred less frequently in their diet. Diets of smelt (Osmerus eperlanus), lesser sandeel (Ammodytes tobianus), juvenile flounder and 0-group turbot were dominated by amphipods and zooplankton. Altogether, the coexistence of the different fish species at the nursery grounds suggests high trophic flexibility and ability of species or size classes of species to partition the available food sources

Stomach analyses of Baltic salmon from 1959-1962 and 1994-1997: possible relations between diet and yolk-sac-fry mortality (M74)

In recent years, Baltic Sea salmon Salmo salar, have suffered high larval mortality (M74) which can be cured by thiamine treatment. Analyses of lung term mortality records (1928 1998) from two salmon hatcheries suggest that before the 1970s M74 did not oc</p