ICES identifies 'substantial changes' in fish distribution

New advice describes shifts in distribution of 16 fish species, with half of these occurring in relation to management areas.

Scientists at the International Council for the Exploration of the Seas (ICES) identified "substantial changes" in fish distribution patterns across the northeast Atlantic in the past 30 years, with hake and mackerel shifting the most.

Answering a request from the European Commission, experts of the Workshop on Fish Distribution Shifts (WKFISHDISH) found 16 out of 21 species examined have shown changes in their distributions since 1985.

Of these, eight species exhibited distribution changes that crossed total allowable catch (TAC) management areas boundaries.

Environmental conditions such as sea temperature, in addition to changes in the distribution and intensity of fishing effort, were found to be strong drivers for these patterns of change.

The evaluation coupled northeast Atlantic trawl data with information from literature to conclude that hake, mackerel, anchovy, cod, herring, horse mackerel, plaice, and common sole had either shifted their relative distribution between different TAC management​ areas or into areas not currently covered by TACs. 

Distributions of a further eight species -- white and black-bellied anglerfish, blue whiting, megrim, sprat, whiting, haddock, and saithe -- were found to have shifted, though these did not affect the TAC areas.

The remaining five species exhibited no change, or the results were inconclusive regarding distribution shifts.

“Hake is one of the species showing the most dramatic changes in distribution,” said Alan Baudron of the University of Aberdeen, co-chair of the WKFISHDISH workshop and invited expert to the advice drafting group.

From its original distribution on the west of the British Isles, it has expanded eastwards into the North Sea.

“The literature suggests that this change is driven by a density-dependent expansion caused by a reduction in fishing pressure and a subsequent increase in biomass: the larger biomass now expands further into the area of suitable habitat defined by temperature and depth.”

A regime shift towards favorable environmental conditions and subsequent higher recruitment success could also have contributed to the increase in hake biomass.

Potential ecological implications of these shifts in distribution include changes in species interactions.

For example the hake expansion into the North Sea could lead to food competition with saithe, both species feed mainly on Norway pout and the increase in hake could be detrimental to saithe, said Baudron.

Also, access to habitats specific to life stages such as nursery grounds might become more limited.

Management implications might include the spatial mismatch between species distribution and quota allocation.

“For instance, the expansion of hake into the North Sea currently has such implications because the relatively small quota allocated to the North Sea no longer matches the regional abundance, resulting in extensive discarding: since hake is caught as part of a mixed demersal fishery, fishers can't avoid catching it and are forced to discard the over-quota catch,” said Baudron.

"This is likely to be an issue once the landings obligation comes into place for demersal fisheries."

It is the recommendation of the workshop that continual monitoring of the distribution is essential for future management.

It is hoped that in the future, ICES will be able to use pelagic survey data as well as possible commercial catch data with high spatial resolution in order to better evaluate and monitor species distribution changes.