Lesser (Raitt's) Sandeel Ammodytes marinus; Raitt, 1934. Family: Ammodytidae
Sandeels are small eel-like fish which swim in large shoals. They are an abundant and important component of food webs in the North Atlantic. They also support the largest fishery in the North Sea, with recent annual landings in the last decade of around a million tonnes. Of the five species of sandeels inhabiting the North Sea, the lesser sandeel, Ammodytes marinus is the most abundant and comprises over 90% of sandeel fishery catches.
Sandeels have a close association with sandy substrates into which they burrow. Observations on the availability of A. marinus to fisheries and their occurrence in sediment suggests that this species rarely emerges from the seabed between September and March, except in December and January when it spawns. Research by Marine Scotland Science has shown that sandeels live in specific types of sand. Tagging studies have indicated that sandeels remain in preferred sandy areas. This dependence on sandy sediments means that the distribution of juvenile and adult sandeels is restricted by the patchiness of their preferred sandy habitat.
Sandeels mature between age one and three. They spawn a single batch of eggs in December-January, several months after ceasing to feed. The eggs are deposited on the seabed. The larvae hatch after several weeks, usually in February-March, and drift in the currents for one to three months, after which they settle on the sandy seabed.
Sandeels are comparatively short-lived with a life span of less than 10 years. They settle after their planktonic phase, at around 4-5 cm length, and may reach 5-10 cm length within three months of hatching. Sandeels off the Firth of Forth are relatively slow growing compared to those in the main fished areas around the Dogger and Fisher Banks. Further, whilst a significant proportion of sandeels from the Fisher, Outer Shoal and Klondyke Banks are likely to spawn at age one, many of those from the Firth of Forth do not spawn until the age of three. Shetland sandeels also tend to be slower growing, although they generally grow faster than Firth of Forth sandeels.
During the active feeding season (April-September) A. marinus tend to emerge during daylight hours to forage close to their burrows. Their main prey is calanoid copepods, but other planktonic prey, including fish larvae, are also taken. Large sandeels may also take benthic prey such as polychaete worms.
Role Within the Marine Ecosystem
Sandeels are an important prey species for many marine predators (such as seabirds and fish). The magnitude of the fishery has led to concern over the potential consequences of sandeel harvesting on the North Sea marine ecosystem. In the North Sea, fishing mortality is lower than natural mortality. Multi-species analyses have shown that, within the scale of the North Sea region, under these conditions, the predators do not suffer a lack of food. However, locally concentrated harvesting, for example targeting one patch more than another, may cause local and temporary depletions of food for predators and, therefore, harvesting should be spread evenly across the fishery area. Changes in the distribution of exploitation during the 1990s heightened concern over possible effects on local sandeel populations.
Managing Sandeel Stocks
The International Council for the Exploration of the Sea (ICES) recommends that local depletion of sandeel aggregations by fisheries should be prevented, particularly in areas where predators congregate. In the light of studies linking low sandeel availability to poor breeding success of kittiwake, ICES advised a closure of the sandeel fisheries east of Scotland for 2000-2003. All commercial fishing was excluded, except for a maximum of 10 boat days in each of May and June for stock monitoring purposes. The closed area is being maintained for three years with an evaluation every year. Marine Scotland Science is involved in providing the advice for this evaluation. Furthermore there are a number of ongoing empirical and modelling investigations of sandeels and interactions between sandeels, predators and the fishery.
Stock management currently treats sandeels in the North Sea as a single population. However, research co-ordinated by Marine Scotland Science indicates that there are a number of stock components in the North Sea which, because of isolation of suitable habitat and limited larval movements, do not inter-mix. Given the potential for differences in growth, recruitment and mortality between these stock components, the present management of the stock by a single Total Allowable Catch (TAC) covering the whole North Sea makes sandeels vulnerable to regional over-exploitation. Scientists from the UK and Denmark are now trying to understand the dynamics of regional stock components in order to provide advice on regional management.
Scottish Sandeel Fisheries
Off Scotland, small sandeel fisheries operate at Shetland and off the west coast. These fisheries are rather different in character to the large North Sea sandeel fishery. They are smaller in scale and restricted to small inshore grounds, and managed nationally. However, it was in relation to the Shetland fishery that concerns were first raised about the potential effects of sandeel fisheries on the availability of sandeels to other predators, making the history of the operation and management of this fishery of more general interest.
The fishery at Shetland started in the early 1970s with the highest landings recorded in 1982 when 52,000 tonnes were landed. Subsequently effort declined as price differences made other fisheries more attractive. Up until 1988 the fishery was unrestricted, but following a few years of low sandeel recruitment and poor seabird breeding success, concern was voiced about the possible adverse effects of the fishery on the availability of sandeels to seabirds. In addition, as the stock had fallen to a low level, some management was considered necessary to provide a measure of protection for the stock. As a result the fishery was closed in 1989 from 1 July to the end of December to restrict fishing effort. This seasonal closure was implemented again in 1990, and in 1991 the fishery was closed completely following analysis which indicated that the spawning stock may have fallen to a level where the probability of high recruitment was reduced.
Although the management of the fishery was reviewed annually, in practice it was not re-opened until 1995. At this time the basis for the management of the fishery had been changed by legislation requiring fisheries management to take account of wildlife conservation, as well as fisheries considerations. The Shetland fishery is not thought to have had a significant effect on the availability of sandeels to seabirds. However, subsequent management of the fishery has explicitly recognised the importance of the Shetland sandeel population to seabirds. The management has also been informed by improved understanding of sandeel population structure. Consultation with the main stakeholders has also been an important aspect of the recent management of the fishery.