[Comment – the following article appeared (2 December 2005) in the ‘Fishing News’, a weekly newspaper serving the commercial fishing industry. We thank both the author, Dr Mike Pawson of Cefas, and the Editor of ‘Fishing News’, for allowing us to publish the article on this web site.]
Sea bass: stocks, fisheries and management around England and Wales
Last week Defra announced a consultation exercise on measures to increase the number and size of bass available to commercial and recreational fisheries. It sought stakeholders’ views on proposals to increase the minimum landing size (MLS) of bass that may be caught in UK fishery limits from 36 to 45 cm. In addition, there was a complementary proposal to increase the mesh size in enmeshing nets targeting bass. In proposing these measures, the government is looking to increase the value of commercial bass landings, to enable more fish to spawn and to increase the number of larger fish available to anglers.
In this article, Mike Pawson from Cefas describes the stock structure of bass exploited around the UK coasts, trends in the fishery and the bass population, and the scientific background to the MLS and mesh size proposals.
The sea areas in which stocks of bass have been identified, based on migratory patterns and exploiting fisheries, are shown in Figure 1. Arrows indicate movements of adult bass within these areas as shown by tagging exercises carried out in the late 1970s – early 1980s and in 2000/3. The results of these studies were similar, except that a migration of bass in late autumn from the North Sea to offshore spawning areas in the western English Channel (where these fish were caught in the pair-trawl fishery in winter) no longer appears to take place. In general, adult bass now appear to remain near their summer feeding areas much later in the year, and to spawn further north, possibly as a consequence of climate warming.
The most recent tagging study also showed that the winter offshore fishery probably accounts for no more than 10% of the exploitation on bass otherwise taken in fisheries (both commercial and recreational) within UK territorial waters. Overall, there appears to be relatively little mixing between “UK” stocks and those along the continental and Irish coasts.
The following table presents estimates of the annual landings of bass (tonnes) taken by UK vessels between 1985 and 2004, by “stock area”. These catch estimates include additional catch obtained from a voluntary, paid log-book system run by Cefas since 1985. This scheme provides daily catch and effort records from a sample of 45-60 small inshore vessels, that are raised to numbers of vessels known to be active in the fishery from an annual fleet census.
It can be seen that bass landings have increased in all areas since 1992, with the largest increase occurring in 1994 when the very strong 1989 year class was spreading throughout coastal waters; and that landings in the last three years have been 2-4 times higher than those in the late 1980s.
An analytical model has been developed by Cefas using data on lengths and ages of sea bass in UK commercial landings by gear group (i.e. trawls, enmeshing nets, hook and line) and stock area for the period 1985 – 2004. Two outputs from this assessment help us to understand how the increase in landings described above may have come about.
First, Figure 2 shows the relative abundance of bass at age 3 in each “stock” area, plus abundance indices of the same year classes obtained from an annual trawl survey in the Solent. Two features stand out. The first is that the pattern of year class strength is very similar in all areas, and also between the catch-derived estimates and the survey indices, which suggests that there is a common factor that determines the survival of young bass. Biological studies have shown that warmer water in the fishes’ first summer boosts their growth, and that cold water in their first winter will lead to higher mortality, and this may explain the second feature, which is that year-class strength has on average been higher since 1989 than previously.
Following the implementation in 1990 of a management package aimed at protecting juvenile bass (a MLS of 36 cm, mesh sizes of 65-89 mm banned in enmeshing nets, and boat-fishing for bass prohibited in “bass nursery areas”), these fish now recruit to the fishery at age 4-6 years old (previously 3-5). Because overall fishing mortality on bass has not increased through this period, this has led to more fish recruiting to the spawning stock, and the fishery, with the result that the spawning stock biomass (SSB, shown in Figure 3), has increased in most areas since the mid-1990s. Our model estimates of SSB are unlikely to be exact in absolute terms, but the adult stocks in the Channel and on the west coast appear to have approximately doubled over the period 1990 – 2004. Whilst there is less evidence that this has happened in the North Sea, bass fisheries there have developed in many areas that are not covered by our log-book system, and we believe that the model is less robust than for the Channel and west coast.
The effects of an increased MLS
In order to achieve the government’s aim, to increase the number and size of fish in the bass piopulation, it would be necessary to protect more bass from exploitation. This can be done either by reducing fishing pressure on bass overall or by directing fishing away from small to medium sized fish. The 1990 bass management package had the same aim (though the objective then was stock conservation), and it succeeded by protecting bass up to 36 cm, allowed more fish to spawn, and achieved a 30-40% higher yield to the fishery for a given level of recruitment (yield per recruit). Because bass are, on average, being caught at larger sizes (than before 1990) and recruitment has increased due to a favourable climate change, especially warmer winters, catches in the fishery have also increased.
We have used the same approach to model the likely effects of a further increase in the size at which bass first recruit to the fishery, based on our current assessment of the bass fishery. This shows that the highest yield per recruit occurs at around 48-50 cm and decreases thereafter, though higher yields could be obtained in some parts of the fishery if fishing effort increased.
If an effective MLS of 45 cm (and a complementary mesh size for enmeshing nets) is imposed in the fishery, this is equivalent to removing all bass of 6 years and younger from the fishery’s catch and leaves many more fish to survive to be bigger than 45 cm. Assuming that fishing effort on bass does not increase, the numbers of the 2002 year class (currently beginning to recruit to the fishery) should be some 30-60 % higher by 2008/9. This increase would be largest if the measures apply effectively to all fishing gears, and least if they only applied to lines (i.e. just a 45 cm MLS).
Though a first exploitation size at 45 cm also confers considerable protection of female bass up to the size at which they first spawn (42-45 cm in NW Europe), the succession of strong years classes in the last 10 years suggests that there is no problem with the stock’s ability to sustain itself.
Because more young bass will survive to be caught when older (and larger), the catch of 7 years and older bass would be increased for all gears. The loss of younger fish, however, means that landings (weight) for trawls, nets and lines will show initial declines but that these recover and will be close to (trawls) or above (nets and lines) the current level after 5 years. This analysis assumes that the larger bass will not be targeted as effectively as smaller fish, but we expect the fishery to adapt rapidly to any new management measures and the recovery in landings to occur that much faster. One benefit, of course, will be that the increase in average size of fish in the catch will attract higher value per kg on the market.
Complementary mesh size controls
In the 1980s, Cefas carried out extensive trials to determine mesh selection characteristics for bass in enmeshing gears (drift and fixed nets), and analytically demonstrated that a minimum gill net mesh size of 90 mm complements an MLS of 36 cm for bass (as currently used in national and EU technical mesh size regulations).
We have used a similar approach, based on relationships between mesh size and the length and maximum girth of fish captured in gill nets, which indicates that 10% of the bass caught in a 107 mm net would be under 45 cm, assuming equal numbers of all sizes encounter the net. The optimum size of capture for this mesh size is 48-49 cm.
A second approach that uses observed length distributions of bass in known mesh sizes (70-92 mm), indicates that a mesh size of approximately 109 mm is needed to obtain a catch where the proportion of bass less than 45 cm is 10%, again assuming equal numbers of all sizes encounter the net.
These two methods give similar results, and suggest that a minimum mesh size (stretched from knot to opposite knot) of between 105 and 110 mm would ensure that the majority of bass caught in enmeshing nets are above 45 cm. There is, nevertheless, some uncertainty in extrapolating such models outside the mesh size range normally used for bass (82-100 mm) for which we have data, and we would welcome the views of the industry as to an appropriate mesh size for a bass MLS of 45 cm.
Figure 1 Stock areas and migration routes of adult bass identified by tagging
Figure 3 Trends in spawning stock biomass (SSB) of bass in stock areas around UK coast, 1985 – 2004
Figure 2 Relative abundance of 3-year-old bass spawned between 1982 and 2003: most reliable estimates between horizontal dashed lines. Solent trawl survey index in black