Stock Dynamics, Interactions and Recruitment in North East Atlantic Squid Fisheries

At the outset of this project the age estimation of squid by reading the incremental structures within the statolith was hardly established in Europe. Pioneered by Japanese and Russian scientists, the methodology received acceptance in Europe at an international workshop held in Sicily in 1989 and published in 1992. These new methods were adopted and evolved by the project and brought to a more rigorous level of standardised preparation and interpretation.

Workshops on preparation and counting of statoliths of loliginid and ommastrephid squids were held in Vigo, Lisbon, Faro and the Azores. Rigorous testing of the methodology confirmed that it could be successfully applied to loliginid squids, and that experienced readers produced consistent repeat counts. Differences between readers remained significant and to achieve standardization, intercalibration of results was found to be essential if results from several laboratories were to be pooled. As a result of this finding, as far as possible this activity was retained within one laboratory group. In the latter stages of the project, the routine use of image analysis equipment has considerably improved consistency and reliability. Statoliths of ommastrephid squids present fewer problems since rings are more clearly defined.

Once reliably established the the aging method was applied to ecological questions arising from population sampling:

These findings have

Almost 18 months of project time in the Azores were used to establish a dedicated system for the holding of live Loligo forbesi. Although continuously in use for the second half of the project, the live-rearing work continued to encounter problems with poor survival of animals, due to a variety of factors relating to capture, transport, water quality, collision of swimming squid with tank walls and agonistic interactions between squid. Additional problems arose from the intermediate seawater holding facility in the harbour (between capture and transport into the aquarium) which became unusable at a critical period due to water pollution.

Of those squid successfully reared in the aquarium for periods up to 42 days only a minority showed positive growth rates. Those successfully marked with tetracycline (to produce a fluorescent band at the growing edge of the statolith) mostly showed poor subsequent growth and survival.

A secondary objective of the live-rearing studies was to provide material to establish a relationship between measured growth (size and weight) and possible indices of growth (RNA concentration). The RNA:protein ratios of all surviving squid did not show significant correlation with a range of negative and positive values for growth rate. However, samples of squid from the field populations showed consistent and highly significant differences in RNA:protein ratios between sexes; between maturity stages; and between species. These values are compatible with other evidence of growth rates.

Important advances have been made in the development of rearing techniques for Loligo spp. and the aquarium facilities and knowledge of squid rearing now in place in the Azores are probably unique in Europe.

Biological samples supporting all objectives of the study were collected from throughout the geographical range of the project from two sources:

(i) commercial sources - squid (Loligo spp) were bought at commercial prices directly from fishing industry sources; fishermen, markets or agents. The cost of this activity was high both in terms of the price to be paid and in staff time and travel. It was, neverthe less, the core supply of specimen material and absolutely essential to the project objectives. These samples were used to provide basic population parameters of size structure, maturity state and specific tissue samples for aging, growth indices and genetics.

For population purposes the sample sizes were smaller than desirable and sometimes prone to selection (by size and condition) by fishermen. To overcome these disadvantages a larger programme of market sampling - statistically defensible and monitoring of size, sex and condition across the whole catch - was instituted in selected ports (e.g. Port-en Bressin, Kinlochbervie). This wide scale, low intensity, sampling for Loligo forbesi has enabled:

(ii) research cruises - throughout the range, incidental capture of cephalopods from research cruises and surveys undertaken by the partner institutions also provided valuable samples. The use by the project of these on-going programmes, primarily directed at other objectives such as young fish recuitment, proved to be a very cost-effective method of obtaining samples and data (see below). The research cruises were also the most useful source of samples of other squid species (Todaropsis eblanae, Illex coindetii, Todarodes sagittatus, Alloteuthis subulata).

In addition to supplying material for the main objectives of the project, a continued programme of biological work was undertaken on the same samples. New reproductive data on fecundity, timing of population spawning, development times and recruitment have been obtained. Extended investigation into the diets of squid has emphasized the degree to which both loliginids and ommastrephids rely on the bottom for sources of food. Related EU funded work on the cephalopod component of marine mammal diet has shown that even in the Northeast Atlantic, cephalopods comprise a significant fraction of the total biomass intake of these populations.

All national partners in the project have continued to compile their regional fishery landing statistics on all cephalopods of commercial potential, and to refine the information recorded. These activities have contributed directly to project objectives (analysis of fishing trends) and also have been supplied to ICES. Partners in this project have been active participants in the ICES Study Group on cephalopods (now Working Group) and have been instrumental in pushing for improvements in the quality of national fishery statistics requested by ICES.

Statistical treatments of these data have been made to investigate whether they may be used as real population abundance estimates; to identify long-term inter-annual trends in landings; to correlate landings by different countries throughout the range; and to evaluate possible relationships with environmental datasets. Progress with these objectives continues to be hampered by the poor quality of data on fisheries available from national sources. In particular, correct species identifications are frequently missing (all countries), and the location of catches (catch-by- square) is only readily available in two EU countries (UK, FR). Our analyses have produced the following provisional conclusions for the loliginid squid:

Despite the deficiences in raw data these analyses are pointing to two very significant results. Firstly, that catch trends may be driven by common factors unlikely to be directly related to fishing activity and, secondly, that oceanographic variables (specifically sea surface temperature) may become a realistic tool for short-term catch forecasting.

The most productive components of the plankton surveys were the research cruises by RV POSEIDON in 1991/92 (historical collections) and 1994/95 (targeted cephalopod cruises with participation by project partners). In these last two EUROSQUID cruises totals of 820 (1994) and 566 (19950 cephalopod paralarvae and juvenile forms were obtained. It is notable that the bulk of these catches were from oceanic and mid-water species groups of little or no commercial significance. Even coastal collections from other sources consistently provide more specimens from these groups than from those commercially important as adults with Loligo only frequent in Bongo net hauls..

These collections and cruise samples have contributed substantially to knowledge of oceanic and mid-water squid families (e.g. Enoploteuthidae, Cranchiidae, Onychoteuthidae) but little information useful to fisheries has emerged.

In contrast, the samples and data from research cruise hauls using fine-meshed fishing nets in coastal waters (young fish surveys) has provided much information on juvenile, pre-recruit sized animals leading to results applicable to fisheries. These quantitative methods have provided:

As well as the value of these samples and data to the interpretation of the biology of Loligo spp., we have shown that pre-recruit abundance in spring surveys is correlated with fishery abundance in the autumn. The method could potentially be applied to fishery forecasting although, presently, variability is too high for practical use. This variability could be overcome using targeted pre-recruit surveys.

Three main aims, and areas of study, were pursued:

1. Preservation and processing of cephalopod tissues for use in biochemical and molecular genetic studies. The main findings were:

2. Species identification and taxonomy using biochemical/molecular methods. The main findings were:

3. Population structuring within NE Atlantic loliginid squid. Loligo forbesi was used as a model for development of methodologies. The main findings were:

The short lived nature of squid species necessitates the use of assessment methods which do not depend on a long time series of data in which cohorts are followed for several years. The assessment methods available for fished species have all been reviewed with respect to their possible application to cephalopod stocks. We suggest that 'depletion' methods offer some scope for progress, although the nature and scale of the fisheries in EU waters mitigate against the use of the detailed procedures adopted in some other areas such as the South Atlantic.

Recruitment is very variable and in some years supports only a very small fishery. Natural mortality remains imprecisely known. The main species of commercial interest are migratory, making it difficult to (a) define suitable stock units and (b) to be sure that observed depletion rates reflect declines in abundance due to fishing rather than immigration and emigration effects. Furthermore the squid are frequently a by-catch in fisheries targeted at other species. This may compound the problem since decline in squid catch rate may reflect a move by the fishermen in response to the abundance of another species. The answer may be to reconsider the 'unit stock' definition and to divide the area for assessment purposes before aggregating the various estimates to produce an overall stock estimate.

In selected areas (North Sea and Rockall), depletion methods produced very satisfactory and robust estimates of population size. The distribution of fishing effort by different fleets was more similar and the fact that squid are a by-catch ensures that the decline in CPUE is a realistic reflection of declining abundance. Depletion estimates of stock size at Rockall (for 1989) were reasonably consistent with the results from swept area surveys in September of the same year, suggesting that of an initial stock of 5-6 millions, of which some 2 million animals remained in September.

The prospects for fishery management can be summarised as follows:

The main practical applications envisaged for the results of this project are in relation to the exploitation, assessment and management of cephalopod resources: through improved understanding of the biology of the fished stocks; through development of methods and acquisition of data for stock assessment; and through pointing to priorities for developments in cephalopod resource exploitation and for new research.

These findings suggest that large scale environmental processes underly the variability of cephalopod populations and point to causal hypotheses which can be tested. The importance of developing empirical models of temporal and spatial patterns in distribution and abundance in relation to environmental conditions, for use in fishery forecasting, is indicated.

These findings suggest that the major part of the European population of Loligo forbesi is to be regarded as a single stock. Special considerations apply to offshore and island populations. The approach is now ready to be applied to other cephalopod species and, coupled to further population and environmental studies, forms the basis for a comprehensive interpretation of the population biology of L.forbesi.

Exploitation plans are described under three headings: sequences and databases (3.1), publications (3.2) and other dissemination activities (3.3).

Microsatellite sequences are being registered with the EMBL/GENBANK sequence databases. This both establishes ownership of the work and will ultimately allow other researchers to have access to the sequences for studies on other cephalopod species.

The cloned Loligo forbesi DNA fragments that were found to contain microsatellite repeat motifs will be entered into the databases. However, public access has been specifically prohibited until the sequences have been published. A total of 54 such sequences were identified, of which at least the 17 most important will be registered. The COIII, 12s rDNA and 28s rDNA sequences extracted will also be registered.

The project has given rise to two substantial databases; on squid life history parameters and on cephalopod fishery statistical data. Both are currently held at the University of Aberdeen and are presently based on Dataease software (Sapphire International plc). It is envisaged that the databases will continue to be held at Aberdeen, facilitating continued maintenance and revision, but the information will be available for consultation by other scientists.

The life-history database contains biological (reproductive, morphometric) data collected from monthly samples of Loligo spp. over the course of the project and the preceding project (FAR MA.1.141) and, to a lesser extent, information on other cephalopods. Ultimately, population data covering up to 6 years (1990-95), for areas from Scotland to the Azores, will be available at one site. The master database at Aberdeen continues to be updated to include recent information from all partners in the project.

The fishery statistical database contains compilations of fishery information on cephalopods from government sources (UK, France, Portugal) and from ICES and FAO. New data continue to be added as available. The highest level of spatial and temporal resolution is seen in the UK and French landings and effort data, which are aggregated by ICES Statistical Rectangle of capture by month. Data from other countries are generally of lower resolution, e.g. by ICES fishery sub-division by year.

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