Why do Fish Migrate?
From the dispersal of eggs and larvae to the eventual return of adults to spawning grounds, the life-histories of many marine fish involve movement or active transport at some stage. Fish migrate because rarely does one habitat provide optimal conditions for all life-history stages. Areas of high food availability are rarely in the same place throughout the year, and often in entirely different habitat to areas that are good for spawning.
Better Opportunities for Growth and Reproduction
Fish achieve better growth rates and greater reproductive success by moving between areas. These movements may be on a grand scale of hundreds or thousands of miles or simply a movement into and out of coastal waters. Most often, migrations are highly seasonal and result in temporal and spatial aggregations of fish.
Why we Need to Understand Fish Migratatory Behaviour
Migration makes fish more vulnerable to exploitation (fishing). This is clearly of significance for commercial fisheries, and fisheries have traditionally exploited this migratory and aggregating behaviour in, for example, salmon and cod. Unfortunately, exploiting a species during its period of reproduction can be most destructive for future recruitment. Understanding migratory behaviour has therefore come to the forefront of management plans that aim for stock recovery and future sustainable management. For migratory species on the verge of stock collapse, we must now aim to maximise the potential to recover. This may involve protection in different areas at different times of the year.
Marine Scotland Science is currently working in collaboration with European partners to investige the migratory behaviour of the Atlantic cod (Gadus morhua). Using state-of-the-art data recording tags (shown below) cod were released from four main areas in the North Sea (CODYSSEY).
What the Data Tags Can Tell Us
Tagged fish are returned by fishermen and we are able to interpret the spatial and temporal migratory patterns from the data tags. These data, together with information on the fish's daily growth estimated from analysis of the fish's ear bone (otolith) will be used to develop an understanding of the timing of movements. It is important to understand if North Sea cod in general have a particular migration pattern or if each local population has its own specific pattern. With this information Marine Scotland Science will be in a better position to suggest positive ways for the future recovery of the stock.