Answering a request from the European Commission, experts of the Workshop on Fish Distribution Shifts (WKFISHDISH) found that 16 out of 21 species examined have shown changes in their distributions across the northeast Atlantic since 1985, with hake and mackerel shifting the most. Of these, eight species exhibited distribution changes that crossed quota management and allocation 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 total allowable catch (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.
Influence of environmental changes
While distribution changes were detailed, it was not possible to evaluate the current relative abundance of each species within each TAC area the
available surveys are not designed to consistent sampling across the regions. The surveys were useful for understanding the distribution of the species across the ICES survey areas as well as changes in their centres of distribution. Large, continuous, and directional changes in distribution determined from the survey data and the literature review were used by the experts to describe 'substantial changes in distribution'.
In addition to identifying the main drivers of distribution changes, environment and fishing, this study also identified the likely mechanisms through which these two drivers impact distribution, such as habitat selection, density-dependence, geographical attachment, or spatial dependency. Environmental changes, mainly characterised in the literature as changes in temperature, influenced the distribution of all 16 species that exhibited substantial changes in distribution, while changes in fishing effort was found to have influenced the distribution of ten of these species.
Alan Baudron of the University of Aberdeen, co-chair of the WKFISHDISH workshop and invited expert to the advice drafting group, provided an example of how these interacting mechanisms helped drive the altered distribution of hake.
"Hake is one of the species showing the most dramatic changes in distribution. From its original distribution on the west of the British Isles, it has expanded eastwards into the North Sea. This change was evident from the trends in occurrence observed within the North Sea ICES areas as well as from the shifts in centre of biomass observed in the North Sea survey data. 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 favourable environmental conditions and subsequent higher recruitment success could also have contributed to the increase in hake biomass."
Changes in species interactions
Global sea temperatures have risen over recent decades, and widely-documented, often northward, shifts in the ranges of various fish species are being linked to these changing environmental conditions. Since 1985, both environmental factors and changes in fishing pressure intensity and distribution have acted as drivers. Although future changes in distribution are likely, predicting how they will play out with any precision is hampered by the difficulty in identifying the associated mechanisms as well as the ability to predict these two drivers. This is further challenged by the fact that the effects of environmental and fishing changes are species- and stock-specific.
It may be, however, that the environment will have a greater impact than fishing: as climate models forecast increasing temperature and primary production, and commercial catches tend to move towards sustainable levels, the relative influence of fishing mortality may be reduced.
As Baudron explained, however, reasonable assumptions can be made about the consequences, both for ecosystems and for management.
"Potential ecological implications 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. 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. This is likely to be an issue once the landings obligation comes into place for demersal fisheries. Such management implications could potentially apply to the other species showing changes in relative distribution across TAC management areas as reported by FISHDISH, although there is no evidence of this yet."
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.
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