A model of meta-population dynamics for North Sea and West of Scotland cod—The dynamic consequences of natal fidelity

Abstract

It is clear from a variety of data that cod (Gadus morhua) in the North Sea do not constitute a homogeneous population that will rapidly redistribute in response to local variability in exploitation. Hence, local exploitation has the potential to deplete local populations, perhaps to the extent that depensation occurs and recovery is impossible without recolonisation from other areas, with consequent loss of genetic diversity. The oceanographic, biological and behavioural processes which maintain the spatial population structures are only partly understood, and one of the key unknown factors is the extent to which cod exhibit homing migrations to natal spawning areas. Here, we describe a model comprising 10 interlinked demes of cod in European waters, each representing groups of fish with a common natal origin. The spawning locations of fish in each deme are governed by a variety of rules concerning oceanographic dispersal, migration behaviour and straying. We describe numerical experiments with the model and comparisons with observations, which lead us to conclude that active homing is probably not necessary to explain some of the population structures of European cod. Separation of some sub-populations is possible through distance and oceanographic processes affecting the dispersal of eggs and larvae. However, other evidence suggests that homing may be a necessary behaviour to explain the structure of other sub-populations. The consequences for fisheries management of taking into account spatial population structuring are complicated. For example, recovery or recolonisation strategies require consideration not only of mortality rates in the target area for restoration, but also in the source areas for the recruits which may be far removed depending on the oceanography. The model has an inbuilt capability to address issues concerning the effects of climate change, including temperature change, on spatial patterns of recruitment, development and population structure in cod.

Theory, consequences and evidence of eroding population spatial structure in harvested marine fishes: a review

“…Schindler et al 2010). Under conditions of locally adapted populations, abundance recovery after a local extinction may take a long time and probably a lot longer than the time estimated by models that do not take into account the spatial and genetic structure of the populations (Hutchings 2000, Heath et al 2008, Murawski 2010. These consequences highlight the relevance of maintaining the 'biocomplexity' of exploited populations.…”

Spatial analysis of North Sea cod recruitment: concurrent effects of changes in spawning stock biomass, temperature and herring abundance

“…However, the young are not spread by random all over the NS (e.g. Heath et al 2008, Gröger et al 2010.…”

“…Based on microsatellite DNA markers, Hutchinson et al (2001) found 4 genetically distinct groups. Advection patterns indicate that cod in the southern NS (the German Bight, the Dogger Bank and the Southern Bight) are quite isolated from cod in the remaining NS (Heath et al 2008). Also, the elemental signature of the otoliths indicates that most adults originate from local nursery areas, and that there does not appear to be detectable exchange between spawning areas > 200 km apart (Wright et al 2006).…”

“…From these data we used the mean spring temperature (March to May, i.e. egg and larval stages of cod; Heath et al 2008) using linear interpolation to quantify the temperature of the centre of each grid cell.…”

“…NS cod is a spatially structured population (Wright et al 2006, Heath et al 2008, making it paramount to take into account the spatial aspects of ecosystem processes (Ciannelli et al 2013, this volume). Also, in order to investigate whether hysteresis is at play, we explored factors that may affect local herring recruitment, including the possible top-down effect of cod on herring.…”