The enhancement of cod stocks

Abstract

Atlantic cod have been a primary target for marine stock enhancement since the 1880s. In the early part of this period, hatched larvae were released in Norway, the USA and Canada. The last larval releases were conducted in Norway in 1971, and a century of cod larvae releases were halted without any clear evidence of benefit. Since the early 1980s, the focus has been on production of larger, more viable juvenile cod. Emphasis has been given to the design of tag–release programmes involving large‐scale releases and ecosystem analysis in selected ecosystems. Most of this research has been carried out in Norway, where more than one million tagged juvenile cod have been released. Smaller stocking experiments have also been performed in Denmark, Sweden, the Faroe Islands and the USA. This paper reviews the major findings from these programmes. We include summaries and evaluations of rearing techniques for juvenile cod, methods of tagging and recapture, experimental fishing, migration, mortality and growth rates in the different habitats, genetic analysis, and ecosystem studies that have tried to describe the variation in the cod carrying capacity of selected release areas. Despite relatively large variation in environmental conditions, in cod production and in fishing mortality along the Norwegian coast, results indicate that, under the conditions experienced during the 1980s and 1990s, releases of juvenile cod did not significantly increase cod production and catches. The biological limitations and future prospects of Atlantic cod stock enhancement are addressed.

How may feeding data be integrated into a model for a Norwegian fjord population of cod (<i>Gadus morhua</i> L.) ?

“…Estimates are based on either feeding data (F) or tagging data (T) in press). Correspondingly, the total biomass of cod of about 2-3 tonnes km -2 in Sørfjord is higher than levels in the 1990s in most other stocks (Svåsand et al, 2000). Thus, the Sørfjord cod stock may be in an "antique" state similar to that of the past early stages of exploitation of other stocks.…”

“…The lowest legal length limit in northern Norway is 47 cm. In southern Norway, much higher levels of fishing mortality and individual growth in coastal cod have been observed (Svåsand et al, 2000). Cannibalism mortality has been proposed to be a possible density-dependent mortality factor that may affect cycling and periodicity in cod stocks (Bjørnstad et al, 1999).…”

Effects of habitat enrichment and food availability on the foraging behaviour of juvenile Atlantic Cod (Gadus morhua L)

“…For example, analysis of long-term data-sets from released larval cod along the Norwegian coast demonstrates little, or limited effect on populations, despite initial effects positively influencing the 0+ class (Salvanes et al 1994;Svasand et al 2000;Chan et al 2003). There are, however, examples where larger juvenile cod have had higher post-release survival compared to smaller fish.…”

Primera liberación de juveniles de mero <i>Epinephelus marginatus</i> (Lowe, 1834) (Serranidae: Teleostei) cultivados en criadero en arrecifes artificiales en el mar Mediterráneo: resultados de un estudio experimental

“…The major factors affecting stocking effectiveness have been demonstrated to be habitat and season, fish size and density at release, fish quality and release technique (Leber et al, 1998;Kristiansen, 1999;Svåsand et al, 2000;Sánchez-Lamadrid, 2002). Indeed, several attempts to enhance wild stocks by means of fish releases failed because of unsuitable habitat and time of release (Bohnsack, 1996), poor knowledge of the biology and ecology of species (Masuda and Tsukamoto, 1998) and low fitness of hatchery-reared fishes due to behavioural and physiological deficits (Munro and Bell, 1997;Howell, 1994).…”