Abstract
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.
Highlights
The changes in hydrographic features and potential changes in cod spawning areas and nursery ground locations over time, together with the resulting recruitment variability and possibly different recruitment regimes [1,2,3,4,5], heavy exploitation by fisheries and likely changes in migration at age between the two Baltic cod stocks [1,6,7] complicate the long-term management of the stocks [8,9,10]
The distribution patterns of juvenile Baltic cod have only been described theoretically via hydrodynamic modelling, with only limited comparisons being made with survey data and fishery observations (e.g., [22,3])
It appears from the present generalised linear model (GLM) analyses, that the smallest juvenile Baltic cod (0–5 cm) occur with the highest densities within the known spawning areas in the Bornholm Basin (SD25), Gdansk Deep (SD26) and parts of the Gotland Basin (SD28), while larger juveniles show the highest densities in more westerly areas in the Arkona Basin (SD24), followed by the Bornholm Basin (SD25)
Summary
The changes in hydrographic features and potential changes in cod spawning areas and nursery ground locations over time, together with the resulting recruitment variability and possibly different recruitment regimes [1,2,3,4,5], heavy exploitation by fisheries and likely changes in migration at age between the two Baltic cod stocks [1,6,7] complicate the long-term management of the stocks [8,9,10]. Extensive long-term fluctuations in stock recruitment have proven to depend on climate-driven hydrographic conditions and regime shifts [15,1,2,3,16,4,6,17,7] All of these factors call for deeper investigations of juvenile cod distribution patterns and variations in the central Baltic Sea. In general, there is a gap in the available scientific knowledge regarding the biology and population dynamics of 0- and 1-group settled juvenile Baltic cod (Gadus morhua) [18,19,3,14,7,17]. The processes and pressures associated with these life stages and the variability in their distribution and abundance patterns as well as their nursery grounds are not well documented in terms of Baltic cod life cycle dynamics [22,3,1,7]
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