Spatio-temporal dynamics of a fish predator: Density-dependent and hydrographic effects on Baltic Sea cod population

Abstract

Understanding the mechanisms of spatial population dynamics is crucial for the successful management of exploited species and ecosystems. However, the underlying mechanisms of spatial distribution are generally complex due to the concurrent forcing of both density-dependent species interactions and density-independent environmental factors. Despite the high economic value and central ecological importance of cod in the Baltic Sea, the drivers of its spatio-temporal population dynamics have not been analytically investigated so far. In this paper, we used an extensive trawl survey dataset in combination with environmental data to investigate the spatial dynamics of the distribution of the Eastern Baltic cod during the past three decades using Generalized Additive Models. The results showed that adult cod distribution was mainly affected by cod population size, and to a minor degree by small-scale hydrological factors and the extent of suitable reproductive areas. As population size decreases, the cod population concentrates to the southern part of the Baltic Sea, where the preferred more marine environment conditions are encountered. Using the fitted models, we predicted the Baltic cod distribution back to the 1970s and a temporal index of cod spatial occupation was developed. Our study will contribute to the management and conservation of this important resource and of the ecosystem where it occurs, by showing the forces shaping its spatial distribution and therefore the potential response of the population to future exploitation and environmental changes.

Spatial contraction of demersal fish populations in a large marine ecosystem

“…The changes in spatial distribution of adult cod are in line with Bartolino et al. () even if the CPUE data used in the historical part (i.e. before 1991) of our analyses, as well as the environmental data and the modelling framework are different.…”

“…In the Baltic Sea, cod ( Gadus morhua ) and flounder ( Platichthys flesus ) are the two dominant species in the demersal fish community (Lindegren, Möllmann, Nielsen, & Stenseth, ; Ustups, Müller‐Karulis, Bergström, Makarchouk, & Šics, ), and both species are of high economic importance. The Eastern Baltic cod stock (hereafter referred to as Baltic cod) has been the focus of many studies throughout the years and information on its spatial dynamics shows that it has experienced a dramatic change in abundance and distribution throughout the last century (Bartolino et al., ; Eero, MacKenzie, Köster, & Gislason, ). Conversely no studies have investigated the long‐term Baltic‐wide spatiotemporal dynamics of flounder.…”

Trophic Interactions, Management Trade-Offs and Climate Change: The Need for Adaptive Thresholds to Operationalize Ecosystem Indicators

“…Fish-based indicators (for the species studied herein) could potentially be more meaningful if they instead are estimated at the broader central Baltic Sea level. The most abundant fish species in the Baltic Sea -which the indicators are based uponmove seasonally and have occupied different ranges over time, mainly regulated by density-dependence (Casini et al, 2011;Bartolino et al, 2017). Our results did neither reveal interactions between climate and fishing pressure as found in previous studies (Gårdmark et al, 2013).…”

“…However, in the Gotland basin, we struggled to identify ecologically sensible models for the two indicators of the apex predator guild, Cod and LPF. This could be due to missing an important covariate or a mismatch between data (indicator or covariate) and spatio-temporal dynamics (Bartolino et al, 2017). Fish-based indicators (for the species studied herein) could potentially be more meaningful if they instead are estimated at the broader central Baltic Sea level.…”

Seasonal dynamics of spatial distributions and overlap between Northeast Arctic cod (Gadus morhua) and capelin (Mallotus villosus) in the Barents Sea

“…We found strong effects of stock biomass on both cod and capelin distributions in autumn, which in particular affected the strength and spatial pattern of overlap between immature cod and capelin ( Fig 8 a, upper panels), but more moderate effects of temperature. Similarly, in the Baltic cod population, stock size had a stronger effect on local cod density than hydrography [ 72 ]. While our study period was characterised by high and relatively stable temperatures [ 26 ], the stock sizes of cod and capelin varied greatly ( Fig 1 ).…”