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Fladen Ground Prawns

Nephrops Changes in the availability of ‘prawns’ at the Fladen Ground

Norway lobsters (Nephrops norvegicus), commonly known as ‘prawns’ and marketed as Scottish langoustines or scampi, are distributed widely around the Scottish coast, supporting the largest fishery for this species in the world with a value to Scotland of £82 million in 2012 (the second most valuable species after mackerel). The distribution of Nephrops is limited to areas of seabed composed of soft mud in which the animal constructs semi-permanent burrows that provide shelter from predators. The single biggest area of suitable mud and the one supporting the largest Nephrops population is the Fladen Ground located in the northern North Sea to the north-east of Scotland, covering an area of nearly 30,000 square kilometres (bigger than the size of Grampian and Perthshire put together).

The Nephrops fishery at the Fladen Ground

International LandingsUnlike some of the smaller inshore prawn grounds which have been fished for around 50 years, the offshore Fladen Ground fishery has developed more recently and has expanded rapidly in the last 20 years or so. Over recent years, landings have regularly exceeded 10,000 tonnes (Figure 1, right) and the area has supported a fleet of larger prawn trawlers.

In 2011 and 2012, however, catches of prawns declined markedly, particularly in the early part of the year. This phenomenon seems to have been repeated in 2013, with only 48% of last year’s prawn landings from the first four months having been landed this year, and industry is concerned about the economic viability of this sector of the fleet. The reasons for the decline in Nephrops catches are complex and arise through a mixture of biological and environmental factors.

Changes in the overall abundance of Nephrops at the Fladen Ground

TV AbundanceThe overall size of the stock potentially has a major influence – it is important to examine how this has changed in recent years. The most recently-published assessment was made by the International Council for the Exploration of the Sea (ICES) in 2012. This assessment is based on underwater television surveys of the Nephrops grounds conducted by Marine Scotland Science (MSS).

These annual surveys provide estimates of the density of Nephrops burrows which are then raised to give an overall stock abundance. Nephrops burrows can be identified by their characteristic shape and counts of these avoid problems associated with the variable emergence patterns of this animal that affect other assessment methods. Observed abundance has fluctuated over the time series of data available, exhibiting a generally rising trend from the late 1990s to 2008 (Figure 2, right). Since then, however, there has been a fairly steep decline to a point just above a ‘trigger’ stock size at which ICES would provide more stringent advice to help maintain the stock in a productive condition. In circumstances of declining abundance, it is clear that catch rates can be expected to decline.

Possible reasons for the observed decline in abundance

What is unclear at present is why the recent declines in overall abundance have occurred. Most populations of fish and shellfish show fluctuations in numbers arising through natural processes affecting the production and survival of young animals that would ultimately recruit to the fishery. Increased mortality through changes in the abundance of predatory species such as cod might also have an effect, although in the complex species mix of the North Sea, attributing declines to any one predator is premature at this stage. Excessive fishing does not seem to be the problem in the Fladen Nephrops fishery since the harvest rates (proportion of the stock removed) are relatively low and the stock is being fished well below the agreed target rates consistent with maximum sustainable yield (MSY).

The distinction between overall abundance and density on the ground

While an understanding of the overall abundance is important, the density of Nephrops on the fishing ground is important too. This factor is often overlooked but it can potentially influence catch rates and the viability of the fishing operation. Although the overall abundance at the Fladen Ground is very high, this is spread over a very large area. The resulting average densities (numbers of Nephrops on a given area of mud) are relatively low and considerably lower than on some other grounds. For example, in 2011 the average density dropped below 0.2 per m2 at Fladen whereas it was around 0.6 per m2 in the Firth of Forth: in other words in an area the size of a small living room you would expect to see two animals at Fladen and six in the Firth of Forth. Fishing in a like-for-like way in the two areas would produce lower catch rates at the Fladen Ground.

The potential effect of the observed lower density on economic viability

During the period of the highest Nephrops landings (2005-2010), average densities at Fladen were close to 0.3 per m2 and this seems to have afforded viable fishing operations. Factors affecting viability in fishing operations are complex and varied and it is difficult to predict an absolute Nephrops density which becomes sub-optimal. Nevertheless, significant reductions at Fladen may be contributing to current difficulties. New assessments from ICES on stock size are expected at the end of June this year: these are based on 2012 TV surveys by MSS and the latter are showing further declines in density. This suggests that the current reduced catch rates will continue for the foreseeable future.

Some of the factors affecting Nephrops behaviour and catch rates

The above observations describe numerical trends in Nephrops without making reference to the day-to-day variability in their availability arising from the behaviour of the species. Nephrops is a burrowing animal which spends variable amounts of time outside the burrow foraging for food or during mating. It is only at these times that Nephrops are available for capture by trawls which skim over the surface leaving the burrows intact beneath the surface of the mud. A wide range of factors affect whether Nephrops comes out of its burrow or not and the patterns observed are very variable. Internal factors such as reproductive state produce different responses between the sexes, and females spend longer in their burrows during egg incubation. External, environmental factors such as light levels at the seabed also influence when the animals emerge, and around Scotland emergence is most commonly timed around dawn and dusk. Similarly, the strength of tides is also known to exert an effect and during spring tides (when local seabed currents are at their strongest) catch rates typically drop. Over the course of a fishing season many of the above factors fluctuate, sometimes cyclically, and are repeated from year to year.

Unusual environmental events may be limiting Nephrops emergence and catch rates

Other environmental factors influence longer term seasonal emergence behaviour and are more closely linked to hydrographic and weather effects. At certain times of the year the development of favourable conditions leads to generally higher emergence rates and fishermen describe this as ‘the prawns coming on’. Temperature plays an important role in influencing feeding behaviour in many marine organisms and in the case of crustaceans like Nephrops, lower temperatures generally reduce activity. Sometimes, unpredictable and more extreme events occur which can interrupt the expected timing of fishing seasons. Recent reports from the east coast of Scotland highlighted very low catches of the closely related lobster (Homarus). The low catches were attributed to the sustained spell of very cold weather and severe winds affecting inshore areas last winter. Similar recent observations were made for Nephrops in Swedish waters of the Skagerrak. Interestingly, against a background of generally rising sea temperatures, measurements suggest that recent North Sea temperatures have been more variable than usual and the annual mean values for the last three years (2010 to 2012) have been lower than in the earlier part of the decade, owing largely to cold winter temperatures. More importantly, International Bottom Trawl Survey (IBTS) measurements showed relatively low bottom temperatures in 2009 to 2011, and specific measurements at the Fladen Ground showed bottom temperatures in the last three years that were lower than the previous decade. Anecdotal information from fishermen has, for a time long, suggested that wind strength and direction may also play a role, and in the Firth of Clyde fishery, sustained easterly winds generally suppress Nephrops catches. Again, the North Sea has experienced unusual weather patterns in the last few years (2009-2011) and a strongly negative North Atlantic Oscillation (NAO) indicates disrupted wind patterns. Over the wider North Sea area, strong, blocking high pressure regions have generated more easterly and northerly winds bringing much colder weather through the winter months, and it is likely that these would affect local hydrodynamics.

Factors combining to influence the Nephrops fishery at the Fladen Ground

It is unclear to what extent individual factors have led to the reduced catch rates observed. What is clear is that there is a complex interplay of these factors and that on top of the reduced average densities in the population, these may have contributed to the reduced availability and catch rates of Nephrops and delayed the normal fishery development.