Being there and getting there: balancing the costs of migration against the benefits of changing habitat for North Sea plaice

To improve survival and reproductive success, many fish species have evolved migratory life-histories, showing ontogenetic and/or seasonal changes in habitat use. Individuals move between different areas, each of which is ‘best’ for a particular activity, such as feeding, growing or spawning. The benefits of moving to a different habitat, however, have to be balanced against the costs of migration, so any behaviour that reduces the cost of movement would be expected to expand the migratory range and thereby increase the range of habitats that can be exploited.

Previous tracking experiments in the southern North Sea have shown that plaice selectively exploit the tidal streams to aid their spawning migration. Here we examine whether this behaviour is primarily (1) an energy-saving strategy, or (2) a transport mechanism by which fish that are unable to navigate over long distances can be carried reliably between feeding and spawning grounds. Because selective tidal stream transport requires that fish remain stationary on the sea-bed during the ‘non-transporting’ tide, energetic calculations predict that this behaviour is beneficial only when the current speed exceeds a critical, size-dependent, value. We have used detailed information about migratory behaviour from individual fish to calculate the metabolic costs and the likely reproductive benefits of different migratory strategies. Our results show that plaice use selective tidal stream transport only in areas where the tidal streams are suitably fast, indicating that this behaviour is primarily an energy saving strategy.