Western Mackerel Research Articles

Anisakis species composition and infection characteristics in Atlantic mackerel, Scomber scombrus, from major European fishing grounds — reflecting changing fish host distribution and migration pattern

Atlantic mackerel (Scomber scombrus) ranks among the most valuable fish species in Europe. The NE Atlantic mackerel population is considered to comprise three main stocks (southern, western and North Sea), with variable proportions of these three intermixing at the northerly feeding grounds. The southern and western mackerel stocks have moved over the past 4–5 years further north- and westward. Consequently, large-scale mackerel fishing and processing have become thriving industries in Iceland and the Faroe Islands in just a few years. The mackerel population structure in the Mediterranean Sea is less well known but seems to comprise at least one, more or less isolated, spawning component. Although mackerel is an important food resource, systematic and concerted epidemiological surveys of Anisakis species in Atlantic mackerel from European fishing grounds have been lacking. As part of the EU FP7 PARASITE project (GA no. 312068), occurrence and specific identity of Anisakis spp. from 1801 mackerel from Northeast Atlantic and Mediterranean waters was investigated. In general, mackerel caught at the Atlantic fishing grounds exhibited markedly higher Anisakis spp. infection levels than fish from the Mediterranean localities. Mackerel caught off NW Spain and Portugal (ICES VIIIc, IXa) showed highest overall and muscular prevalence, reaching 87% and 52%, respectively, which differed significantly from all other Atlantic samples. Lowest overall Anisakis spp. prevalence and abundance was recorded in mackerel from Faroe Isles waters, while lowest muscular infection levels were found in the samples from the North Sea. Genetic nematode species identification showed that A. simplex (sensu stricto) is the dominating species in mackerel from the Atlantic areas, while A. pegreffii dominated in the samples from the Mediterranean Sea. The latter species showed generally low prevalence and intensity in the flesh, not exceeding 6% and one larva, respectively. While A. simplex (s. s.) and A. pegreffii seem to co-occur in mackerel from off NW Spain and Portugal, several A. pegreffii were also recorded in mackerel from the North- and Norwegian Seas. These findings imply that the actual mackerel started their feeding migration in waters south to the British Isles, which include parts of the sympatric area of the two sibling species. Thus, A. pegreffii could prove a useful supplementary marker to track migration routes of the different mackerel stock components in NE Atlantic waters.

A linked separable–ADAPT VPA assessment model for western horse mackerel (Trachurus trachurus), accounting for realized fecundity as a function of fish weight

Abstract De Oliveira, J. A. A., Darby, C. D., and Roel, B. A. 2010. A linked separable–ADAPT VPA assessment model for western horse mackerel (Trachurus trachurus), accounting for realized fecundity as a function of fish weight. – ICES Journal of Marine Science, 67: 916–930. The western horse mackerel stock covers a large area of the Northeast Atlantic and is characterized by the sporadic occurrence of strong year classes. The only fishery-independent data available for its assessments are the estimates of total annual egg production from triennial egg surveys. Horse mackerel are indeterminate spawners, so a direct conversion of total annual egg production to spawning-stock biomass using prespawning-season estimates of fecundity is not viable. There is also evidence that potential fecundity per kg spawning female increases with fish weight. A linked separable–ADAPT VPA (SAD) model was developed that combines data on total catch, catch-at-age, and total annual egg production with data on potential and realized fecundity to provide population estimates for western horse mackerel. The model accounts for potential fecundity as a function of fish weight and for the development of a targeted fishery on the strong 1982 year class. Simulations confirm that the SAD model is able to reproduce population estimates without bias under a range of scenarios, except where there is a trend in realized fecundity. This underscores the need for improved information on realized fecundity, or alternatively the need to develop management plans that are robust to this source of uncertainty.

Investigating the use of proxies for fecundity to improve management advice for western horse mackerel Trachurus trachurus

Abstract Observations of fecundity from the 2001 western horse mackerel spawning-stock biomass survey suggest that the species is an indeterminate spawner. Therefore, estimates of fecundity based on biological analyses and until recently used in the calibration of the stock assessment are now questioned. The stock is assessed by fitting a linked Separable and ADAPT VPA-based model to the catch-at-age data and to the egg production estimates. Currently, the assumption is that egg production and spawning-stock biomass are linked by a constant but unknown fecundity parameter, estimated within the model. In this study, the effects of introducing relationships linking biological indicators of fecundity, such as lipid content or feeding intensity during the spawning season, to actual fecundity are examined within a simulation framework. Simulations suggest that when the underlying relationships between fecundity and the proxy are poorly described, weak, or based on a relatively short time-series of data, the assumption of constant fecundity will result in better management advice than using the proxy.

Migration of adult mackerel along the Atlantic European shelf edge from a tagging experiment in the south of the Bay of Biscay in 1994

Within the frame-work of the Shelf Edge Fisheries Oceanography Study (EU AIR SEFOS Project), 10 000 mackerel were tagged in March and April 1994, off the Basque Country (Spain), at the eastern part of ICES Division VIIIc, close to the boundary between the, so-called, Southern and Western mackerel stocks in the north-east Atlantic. The purpose of this experiment was to outline the migration routes for mackerel spawning in the south-east corner of the Bay of Biscay and to examine the justification for separating a Southern and Western stock. Recaptures were recorded along the western coasts of Europe, from the north-west of Spain to the Norwegian coast. These results connected the fisheries of the southern and north-western areas and put in doubt the existence of two separate stocks in these regions. The paper concludes that the fishery on spawning mackerel that takes place in the east and central Cantabrian sea in the first half of every year is made on mackerel coming from the north-western area extending their southward spawning migration through ICES Division VIIIc, before going back to north. This migration pattern provides a simple explanation for the strong seasonality of the southern fisheries on adult mackerel and for the disappearance of those adults from that area during the second half of the year.

Hydrography and mackerel distribution on the shelf edge west of the Norwegian deeps

A combined acoustic, fishing and hydrographic survey was carried out in the Viking Bank area of the North Sea in December 1995. The aim of the survey was to locate and map the distribution of the western mackerel in the area prior to the start of their spawning migration and to determine the hydrographic background. Large numbers of mackerel were found in greater than 200 m of water on the western edge of the Norwegian Trench. The area was defined by cool waters to the east, south and west, suggesting that the distribution was modulated by water temperature. Data from CTD sections, sea surface temperatures and current meter moorings are presented and the implications for the distribution and migration of the western mackerel are discussed.

On the role of sea surface temperature on the spatial distribution of early stages of mackerel using inertiograms

From a statistical point of view, the relationship between larval density and a physical parameter such as temperature can be described by a regression. Density is then considered as a response to temperature, but the regression depends on the domain (for instance favourable temperatures may extend beyond areas where larvae are present). An alternative approach starts with the individual larva and looks at the associated parameter. The relative abundance per class of parameter gives the probability distribution of the parameter per individual, with its mean (centre of gravity) and variance (inertia). The regression or the distribution of the parameter per individual is often bell-shaped, indicating a maximum. However, this may be caused by the gregarious behaviour of the larvae, onto a relatively smooth parameter field rather than by an actual link between the variables. To answer this question, the spatial context can be used: in case of an actual association, this association is expected to get weaker when moving the population as a whole on the parameter field, resulting in an increase in the inertia of the parameter (inertiogram). The method is illustrated for Western mackerel egg and larval distributions observed during the ICES triennial surveys in 1989. It is shown that, along the shelf edge, (the inertia of) temperature is conditioning the latitudinal extension of the larval populations. D'un point de vue statistique, le lien entre, par exemple, une densité de larves et un paramètre physique comme la température de surface peut être décrit par une régression bivariable. Dans ce cas, on considère que la densité est une réponse à un certain niveau de température. Mais une régression dépend du domaine de calcul (en particulier, on trouve des températures favorables en dehors de la zone de présence des poissons). Une alternative consiste à prendre un individu au hasard et à s'intéresser à la température à laquelle il se trouve. L'abondance relative d'individus par classe de température donne alors la loi de probabilité de la température par individu, ainsi que sa moyenne (centre de gravité) et sa variance (inertie). Régressions et distributions de probabilité ont souvent une forme en cloche et passent donc par un maximum. Toutefois, ceci peut s'expliquer par le comportement grégaire d'une espèce relativement à la structure spatiale assez régulière du paramètre plutôt que par l'existence d'un lien entre les variables. Pour trancher cette question, on suggère de se placer dans un contexte spatial: à supposer qu'un lien existe, le déplacement de la population dans son ensemble devrait détériorer la correspondance entre les distributions spatiales, de la population d'une part, et du paramètre d'autre part, augmentant ainsi l'inertie du paramètre (inertiogramme). L'application concerne ici le rôle de la température de surface sur les oeufs et larves de maquereau observés lors de la campagne triennale du CIEM en 1989. On montre ainsi que, le long des accores, (l'inertie de) la température conditionne l'extension en latitude de la population de larves.

Plankton sample colour as a determinant of the abundance of stage I eggs in the western mackerel (Scomber scombrusL.) stock

Plankton samples were collected during the 1995 International Mackerel Egg Survey. Although the colour of plankton samples was not routinely recorded in sample analysis for mackerel eggs, it was noted that these samples could be consistently categorized as being either white, pink, green or brown. Samples of all colours were found in each of the delineated sea areas of the survey. When the numbers of stage I eggs per unit volume of plankton from the different coloured samples were compared, the pink samples were found to contain over four times as many eggs per unit volume of plankton as any of the other colours. The reasons for this and the possible implications for assessment and management of the mackerel stock are discussed.

Improving the precision of the daily egg production method using generalized additive models

Generalized additive models (GAMs) are used to model spatial variation in egg density and increase the precision of biomass estimates from the daily egg production method. Application of GAMs to survey data from the western mackerel (Scomber scombrus) and horse mackerel (Trachurus trachurus) stocks result in a substantial reduction in coefficients of variation of egg abundance. In developing GAM methods for the daily egg production method, we generalize Pennington's method, in which presence-absence is modelled separately from nonzero observations, and use a new form of the bootstrap that accommodates clustered count data without requiring explicit modelling of the form of clustering. In addition to increasing estimation precision, the use of GAMs has several advantages over stratified sample survey methods. To a large degree they allow the data to determine the form of functional dependence of the response on explanatory variables; they accommodate a wide variety of forms of stochastic variation of the response; they provide maps of the predicted density within the survey area; they provide an objective means of interpolating into unsampled areas; and estimation does not assume random sampling with respect to location.

Recruitment success of different fish stocks in the North Sea in relation to climate variability

Long-term data of year class strengths of different commercially harvested fish stocks based on a virtual population analysis (VPA) are available from ICES. The anomalies of these long-term data sets of year class strength are analyzed using Empirical Orthogonal Functions (EOFs) and are related to climate variability: the anomalies of the sea surface temperature (SST) in the northern North Sea and the North Atlantic Oscillation (NAO) index. A Canonical Correlation Analysis (CCA) between the leading eigenmodes is performed. The results suggest that the variability in the fish recruitment of western mackerel and three gadoids, namely North Sea cod, North Sea saithe, and North Sea whiting is highly correlated to the variability of the North Sea SST which is directly influenced by the NAO. For North Sea haddock and herring no meaningful correlation exists to North Sea SST and NAO.

Cross-shelf processes north of Scotland in relation to the southerly migration of Western mackerel

A combined acoustic and hydrographic survey was conducted west of Shetland in January 1995. The temperature‐salinity structure at the shelf edge north of Scotland was characterised by a narrow (30 km) core of warm, saline water embedded within a broader distribution of Atlantic Water; this would generally mark the area of the shelf-edge current. Current measurements recorded during the period of this survey demonstrated that, uncharacteristically, the core did not mark the area of maximum transport along the shelf break but lay inshore of it. Hence larger scale processes associated with the north-west European shelf edge are important in determining the intermediate scale physical environment encountered by mackerel during their southerly migration to the spawning areas. Acoustic survey data revealed that a large number of the mackerel schools were located in, or close to, this warm saline core at the shelf edge, the remainder being found further inshore. Mackerel school structure varied dramatically between the areas aVected by this core and other parts of the survey area. In general, mackerel form large distinct schools in mid-water, and these were seen during the survey in the shelf waters. In the area of the warm core, schools were found deeper, were more diVuse and tended to form elongated thin layers. We present the hypothesis that the change in schooling behaviour reflects whether or not the schools are actively migrating, and that those schools observed in the warm water core were stationary, and those in cooler waters were actively migrating. ? 1997 International Council for the Exploration of the Sea

Sustainable Yield Analysis in a Multicohort Single-species Fishery: A Mathematical Programming Approach

The concept of sustainable yield, i.e. the fish catch that can be maintained in the long run from a fishery in a steady state, is widely used in fisheries management. In traditional methods of sustainable yield analysis, based on the Schaefer model of a fish stock, the age structure of the stock is ignored. Approaches based on the Beverton-Holt multicohort fish population model take account of age structure but assume that instantaneous natural and fishing mortality rates are constant throughout the year. Using a fish population model in which this assumption is not required, a mixed integer programming model is developed for the analysis of a multicohort single-species fishery in a steady state. This new method of sustainable yield analysis is demonstrated using data for the western mackerel fishery. Comparisons with results from other studies of this fishery are presented.

Is the fecundity of the Atlantic mackerel (Scomber scombrus: Scombridae) determinate?

Abstract Current methods of stock assessment using plankton surveys and fecundity measurements are based on the assumption that the western mackerel (Scomber scombrus L.) has a determinate fecundity, that is, the number of oocytes destined to be matured in a season is decided prior to spawning and can be identified and counted before release of the first egg batch. This assumption has been questioned in recent years and the possibility of a serious miscalculation of fecundity has been raised. Evidence presented here shows that: A hiatus does not develop in the size frequency distribution between the previtellogenic and vitellogenic oocytes prior to spawning. The number of vitellogenic oocytes (potential annual fecundity) declines during spawning. The agreement between the potential annual fecundity measured prior to spawning and the number of eggs shed during spawning calculated by estimating the size and frequency of egg batches and the duration of the spawning season is somewhat ambiguous. The productio...

Optimal Annual Changes in Harvests from Multicohort Fish Stocks: The Case of Western Mackerel

An optimising model is developed to determine the sequence of total allowable catches of western mackerel which would result in the maximum present value of the stream of annual rents. It is found that the optimal steady state fishing mortality would be one half to one third of recent levels. For the base formulation of the model the optimal plan calls for periodic fishing, with a moratorium on fishing for the first two years, and for a very substantial catch in the third year. The sensitivity of pulse fishing as an optimal strategy is tested with respect to number of solution iterations, asymmetric charges for increasing fleet capacity, quadratic adjustment costs, harvesting stock effects, the rate of discount, and the price elasticity of demand for mackerel.

Batch fecundity of Atlantic mackerel, Scomber scombrus L.

The total annual fecundity of mackerel, Scomber scombrus L., during a spawning season is produced by release of discrete batches of eggs. Total fecundity is then the product of the number of batches times the batch fecundity (Fbw). An extensive trawl survey of the Western mackerel stock in the NE Atlantic was undertaken in the 1989 spawning season. Batch fecundity was estimated for 298 fish by counting hydrated eggs in ovaries just prior to ovulation. Fbw, expressed as eggs g−1 body weight, varied with latitude: south of 51°N, Fbw= 55.49, s.e.= 2.04, n= 227; between 51°N and 55°N, Fbw= 45.72, s.e.= 3.41, n= 52 and north of 55° N, Fbw= 41.33, s.e.= 5.52, n= 19. It is suggested that as spawning fish migrate northwards the batch size decreases with progress of the spawning season.

Mackerel (Scomber scombrus) egg mortality: the Western mackerel stock in Biscay and the Western Approaches in 1977, 1980, 1983 and 1986

Mackerel (<i>Scomber scombrus</i>) egg mortality: the Western mackerel stock in Biscay and the Western Approaches in 1977, 1980, 1983 and 1986

Changes in age--size distribution and sex ratio during spawning and migration of Western mackerel (Scomber scombrus L.)

The migrations of the Western stock of mackerel (S. scombrus L.) to the spawning, feeding, overwintering, and back to the spawning grounds are interpreted with reference to changes in mean length, mean weight, mean length-at-age, mean weight-at-age, and sex ratio. Migration to the spawning grounds occurs in an age—size succession. In fish of the same age and size, the males start spawning earlier than the females, while spawning ends at about the same time for both sexes. The return migration after spawning occurs only in an age–size succession. Nearly the whole spawning stock of Western mackerel is gathered in the spawning area by the end of April and the beginning of May, except for the two-year-olds. Larger fish of a given age group arrive earlier in any area and leave it earlier than the smaller fish, which results in a decrease in mean length-at-age and mean weightat-age in all areas where mackerel appear and disappear again. The “fast” and “slow” growing components are the result of the migrations of the age groups in a length succession; they are not based on two subpopulations or populations. The mean length of female mackerel in maturity stage 6 (hyaline eggs present in the ovary) during all months of the spawning season proved to be higher than the mean length of males in this stage: females in maturity stage 6 should be used forstock-size estimation by calculating the mean fecundity at the mid-points of the egg-survey cruises. During the spawning season different sex ratios are observed, which are necessary to calculate the number of males and females in the spawning area on the basis of the number of spawning females which are derived from the egg production. The male/female ratio for the whole Western mackerel stock was estimated as one male per female, but a slight deviation could be caused by the two-year-old males' maturing at a smaller length than the females; the size of this deviation will therefore depend on the contemporary abundance of the two-year-old fish. Throughout the whole spawning eason the male/female ratio of maturity stage 6 mackerel was 56/44, which indicates a longer duration in maturity stage 6 for males than females. The testes seemed to develop earlier than the ovaries, but more slowly.

Changes in Western mackerel (Scomber scombrus) spawning stock composition during the spawning season

INTRODUCTIONTwo groups of mackerel (Scomber scombrus L.) are recognised by the International Council for the Exploration of the Sea (I.C.E.S.) for stock assessment purposes in the north-east Atlantic. The North Sea ‘stock’, which overwinters along the edge of the Norwegian Trench and spawns off the south coast of Norway, in the Skagerrak, Kattegat and the central North Sea (Hamre, 1980), and the Western ‘stock’, which overwinters and spawns along the edge of the continental shelf from the west of Ireland to the Bay of Biscay (Lockwood, Nichols &amp; Dawson, 1981).

An assessment of the Western mackerel stock

An assessment of the Western mackerel stock

Mackerel and Sunshine

In his paper on “Plankton Studies in Eelation to the Western Mackerel Fishery,” in the last number of this Journal (Vol. VIII., p. 269), G. E. Bullen shows that for the years 1903–1907 there appears to be a correlation between the number of mackerel taken during May and the amount of Copepod plankton, upon which the mackerel feed, taken in the neighbourhood of the mackerel fishing grounds during the same month.