Oceanographic influences on the distribution of Baltic cod, Gadus morhua, during spawning in the Bornholm Basin of the Baltic Sea

Abstract

The Baltic Sea is a stratified, semienclosed sea typified by a low‐salinity surface layer and a deep saline layer of varying volume, salinity, temperature and oxygen concentration. The relationships between these oceanographic factors and the distribution of Baltic cod are presented, utilizing results from a survey carried out during the 1995 spawning period in the Bornholm Basin, at present the main spawning area of this stock. Cod distribution, abundance and population structure were estimated from hydroacoustic and trawl data and related to hydrographic parameters as well as to bottom depth. In the central basin, cod were aggregated in an intermediate layer about 15 m thick. This area of peak abundance was defined at its upper limit by the halocline and at the lower limit by oxygen content. The majority of individuals caught in the basin centre were in spawning or pre‐spawning condition with a high proportion of males to females. On the basin slopes, aggregations of cod were found near the bottom. These individuals were mainly immature and maturing stages with an increasing proportion of females to males with size. Salinity and oxygen conditions were found to be the major factors influencing the vertical and horizontal distribution of adult cod. Abundance of immature cod was also positively related to decreasing bottom depths. The effect of temperature was minor. The observed size‐ and sex‐dependent spawning aggregation patterns, in association with habitat volume and stock size, may influence cod catchability and thereby the assessment and exploitation patterns of this stock.

Individual behaviour of Baltic cod Gadus morhua in relation to sex and reproductive state

“…This study confirmed that during maturation stages cod preferred shallow water and did not undertake substantial vertical migrations during this period. Further, it is well known that during summer months, spawning cod aggregate (and hence occur in great abundance) in the deep part of Bornholm Basin (Tomkiewicz et al 1998, Schaber et al 2009. This was likewise confirmed by the present data as all but 2 individuals with gonadal indications of spawning migrated towards deep and saline waters.…”

“…This is also the case with Baltic cod Gadus morhua. Most information available in previous studies describing the 'whereabouts' of Baltic cod has been obtained from trawl or hydroacoustic surveys (Tomkiewicz et al 1998, Scha ber et al 2009. These studies are important in understanding the distributional characteristics of cod, however they do have the limitation of providing observations at a single point in time.…”

“…The employment of archival tags, also known as data storage tags (DSTs), has proven a rather powerful tool for obtaining measurements of variables such as pressure (as a measure of depth) and temperature over extended time scales, records from which information about location and movement of individuals can be inferred (Neat et al 2006, Shepard et al 2006, Hobson et al 2007, Svedäng et al 2007). Essentially, movements of individuals are related to gradients of environmental parameters (Neuenfeldt et al 2007) Despite some knowledge on movement patterns of cod during the spawning season, which indicate that fish move from shallow areas with low salinity into the deep, high-saline parts of the Bornholm Basin (Aro 1989, Tomkiewicz et al 1998, Neuenfeldt et al 2007; individual timing and duration, and whether there are any genderspecific differences in these, are not known. Intuitively, reproductive phases may well correlate with the onset, as well as the termination, of the spawning migration.…”

Feasibility of removal of dioxin and dioxin-like PCB's by intensive fishery of herring and sprat in the Baltic Sea

“…Similar levels were reported for Danish sprat fisheries in May 2003(ICES 2004. Bycatch of cod is probably potentially highest when oxygen conditions in the deep layer are too low, so that sub-adult and adult cod occupy higher levels in the water column or shallow areas (Tomkiewicz et al 1998). In both cases, their overlap of these size groups with sprat and herring could increase (Neuenfeldt papers), thereby increasing the potential for cod to be captured as bycatch in intensive clupeid fisheries.…”

Scientific knowledge of biological processes potentially useful in fish stock predictions

“…Investigations on habitat preferences and threshold values in the Baltic (Tomkiewicz et al, 1998;Neuenfeldt, 2002), suggested that the adult cod stock concentrates in areas with salinities of >10 PSU. When these salinities are not available or not accessible due to low oxygen concentrations in the bottom water as occurring in the eastern Central Baltic (ICES Sub-division 26 and 28), cod prefer at least salinities of >8 PSU.…”