Developing Baltic cod recruitment models. II. Incorporation of environmental variability and species interaction

Abstract

Abstract:We investigate whether a process-oriented approach based on the results of field, laboratory, and modelling studies can be used to develop a stock-environment-recruitment model for Central Baltic cod (Gadus morhua). Based on exploratory statistical analysis, significant variables influencing survival of early life stages and varying systematically among spawning sites were incorporated into stock-recruitment models, first for major cod spawning sites and then combined for the entire Central Baltic. Variables identified included potential egg production by the spawning stock, abiotic conditions affecting survival of eggs, predation by clupeids on eggs, larval transport, and cannibalism. Results showed that recruitment in the most important spawning area, the Bornholm Basin, during 1976-1995 was related to egg production; however, other factors affecting survival of the eggs (oxygen conditions, predation) were also significant and when incorporated explained 69% of the variation in 0-group recruitment. In other spawning areas, variable hydrographic conditions did not allow for regular successful egg development. Hence, relatively simple models proved sufficient to predict recruitment of 0-group cod in these areas, suggesting that key biotic and abiotic processes can be successfully incorporated into recruitment models.Résumé : Nous cherchons à savoir si une approche axée sur les processus et basée sur des études de terrain, de laboratoire et de modélisation peut servir à développer un modèle stock-milieu-recrutement pour la morue franche (Gadus morhua) de la Baltique centrale. À la suite d'une analyse statistique exploratoire, des variables significatives qui influençaient la survie des premiers stades et qui variaient systématiquement d'une site de fraie à un autre ont été incorporées dans des modèles de stock-recrutement, d'abord pour les sites de fraie les plus importants et ensuite combinés pour toute la Baltique centrale. Les variables identifiées comprennent la production potentielle d'oeufs par le stock de géniteurs, les conditions abiotiques qui affectent la survie des oeufs, la prédation des oeufs par les clupéidés, le transport des larves et le cannibalisme. Les résultats indiquent que le recrutement durant la période 1976 à 1995 au site de fraie le plus important, le bassin de Bornholm, était en relation avec la production d'oeufs; cependant, d'autres facteurs significatifs affectaient la survie des oeufs (oxygénation, prédation) et, une fois incorporés, ils expliquaient 69% de la variation dans le recrutement du groupe 0. À d'autres sites de fraie, les conditions hydrographiques variables ne permettaient pas un développement normal et réussi des oeufs. Ainsi, des modèles relativement simples se sont avérés adéquats pour prédire le recrutement des morues du groupe 0 dans ces sites, ce qui laisse croire que des processus dominants abiotiques et biotiques peuvent être incorporés avec succès dans les modèles de recrutement.[Traduit par la Rédaction] Köster et al. 1556

Survival probability of larval sprat in response to decadal changes in diel vertical migration behavior and prey abundance in the Baltic Sea

“…Kö ster et al (2003) reported significant relationships between covariances of abundance estimates between early and late egg stages and between the late larval and the early juvenile stages but no significant relationship between year-class strength and larval abundance. These findings suggested that processes acting during the late larval and early juvenile phase were critical to year-class success, a finding recently shared by Baumann et al (2006).…”

“…Oxygen content and salinity have been shown to be the major environmental factors affecting the Baltic cod egg fertilization and survival rates (Wieland and Zuzarte 1991), and indirectly by determining the vertical overlap of cod eggs and their clupeid predators (Kö ster et al 2001). The principal mechanism influencing the replenishment of oxygen in the deep basins of the Baltic is the inflow of saline oxygen-rich waters from the North Sea.…”

“…Previous studies have found significant but weak correlations between sprat recruitment strength and spawning stock biomass and temperature conditions experienced during the egg and early larval stages of Baltic sprat . However, Kö ster et al (2003) noted that estimates of egg and larval abundance were generally well correlated, whereas larval production poorly predicts recruitment levels. Thus, it has been argued that year-class strength in Baltic sprat may rather be determined by the influence of environmental factors acting during the late larval and early juvenile stages .…”

“…In this species-poor system, predation is unlikely to be a driving factor for a selection process acting on larval fish because of a relatively low encounter rate between piscivores and their prey (Kö ster et al 2001). From a fitness perspective, larval fish should avoid habitats (e.g., water depths) that increase the probability of encounters with predators.…”

“…Haslob et al (2007) could not find any evidence for predatory effects of the new invasive ctenophore Mnemiopsis leidyi on larval fish in the Bornholm Basin. The low degree of fish and invertebrate predation on larval fish in the Baltic Sea likely results from little vertical overlap between predators and prey (Kö ster et al 2001).…”