Groundfish Surveys Research Articles

Variability in maturity and growth in a heavily exploited stock: cod (Gadus morhua L.) in the Irish Sea

Abstract Maturity, sex ratio (proportion female by number) and length-at-age of cod in the Irish Sea were examined using data collected during groundfish surveys in spring 1992–2002. Skewed sex ratios with a predominance of males were observed in the survey catches of 2- and 3-year-old cod taken on the spawning grounds. In contrast, commercial midwater trawl catches of cod in the same area had consistently higher sex ratios, suggesting that mature males and females have different vertical migration behaviour. Estimates of proportion mature were not affected by method of capture. In a GLM analysis, the factor age explained most of the variation in maturity in each sex, whilst length, although a significant factor, explained less variation than year and region (spawning and non-spawning grounds). Maturity was observed in a variable proportion of 2-year-olds, whilst virtually no 1-year-olds and all 3-year-olds and older were mature irrespective of area or year of capture. Population estimates of proportion mature in 2-year-olds, from all years' data combined, were 0.87 in males and 0.45 in females. Estimates were higher in spawning areas than in non-spawning areas, and showed a general increase over time throughout the survey area. However, in any year, the proportion mature at age 2 varied little with length and was reduced only in the smallest fish (<40 cm) at this age in some years. The increase over time in proportion mature coincided with rising sea surface temperature (SST) and a decline in recruitment and stock biomass at high rates of fishing mortality. Mean length-at-age differed consistently between year classes, the differences being largely established in the first year of life. The fastest growth rates were apparent in very weak year classes produced in the late 1990s when SST was relatively high. The interrelationships between growth, maturity, SST and abundance are examined, and changes in growth and maturity since earlier studies in the 1970s are investigated.

Different surveys provide similar pictures of trends in a marine fish community but not of individual fish populations

Abstract Choice of sampling method and survey period can have an important impact on the perception of the structure and dynamics of an ecological community. For the Celtic Sea fish assemblage we compared data obtained by three different trawl surveys: an autumn groundfish survey with a GOV trawl, and a spring and an autumn groundfish survey, both carried out with a Portuguese high-headline trawl. Time-series of abundance estimates were not consistent among surveys for all species and were generally very noisy. An analysis of variance components showed that the sampling method contributed more to the variance in abundance estimates compared to survey period, interannual variability, or even sampling variance. Overall community assessments based on indicators such as proportions of non-commercial and piscivorous species, and the proportion of benthic species showed similar trends for all data series. The shape of the size spectrum based on abundances per length class summed over all fish species, although stable over time, was highly sensitive to the sampling method. With the exception of size spectra, community indicators for marine fish assemblages monitored by surveys seem to be robust to survey period and trawling gear, but species abundance trends are method dependent.

Does diet in Celtic Sea fishes reflect prey availability?

Feeding preferences of Celtic Sea fishes were investigated using a database of stomach content records, collected between 1977 and 1994. The diet of cod Gadus morhua, hake Merluccius merluccius, megrim Lepidorhombus whiffiagonis, whiting Merlangius merlangus and saithe Pollachius virens changed markedly as the animals grew larger, and although large predators generally chose larger bodied prey, the variability of prey sizes consumed also increased. Large predators continued to select small, low value, benthic prey (e.g. Callionymus spp. and Trisopterus spp.) which were easier to catch, rather than larger, more energy lucrative pelagic prey (e.g. mackerel Scomber scombrus), even though these pelagic prey‐fishes were nearly always available and were often very abundant. Stock estimates of the International Council for the Exploration of the Sea and U.K. groundfish survey catches were used as indices of prey abundance. Blue‐whiting Micromesistius poutassou and other small pelagic fishes (Argentina spp. and clupeoids) were identified as being particularly important, and were consumed by some predators more often than would be expected given the abundance of these prey in the environment. There was no evidence for density‐dependent feeding by predators on mackerel and only hake exhibited density‐dependent feeding on horse‐mackerel. Hake, cod and megrim consumed more blue‐whiting when this prey was at higher abundance in the environment. In choosing what prey to consume, predators must balance costs and benefits, considering the quality of prey and the energy expended during search, capture and handling.

Ecological relevance of temporal stability in regional fish catches

The concept of habitat selection based on ‘Ideal Free Distribution’ theory suggests that areas of high suitability may attract larger quantities of fishes than less suitable or unsuitable areas. Catch data were used from groundfish surveys to identify areas of consistently high densities of whiting Merlangius merlangus, cod Gadus morhua and haddock Melanogrammus aeglefinus in the Irish Sea and plaice Pleuronectes platessa, sole Solea solea, lemon sole Microstomus kitt in the English Channel over a period of 10 and 9 years respectively. A method was introduced to delineate areas of the seabed that held consistently high numbers of fishes objectively from large datasets. These areas may constitute important habitat characteristics which may merit further scientific investigations in respect to ‘Essential Fish Habitats’(EFH). In addition, the number of stations with consistently high abundances of fishes and the number of stations where no fishes were caught gave an indication of the site specificity of the fish species analysed. For the gadoids, whiting was found to be less site specific than cod and haddock, while for the flatfishes, plaice and sole were less site specific than lemon sole. The findings are discussed in the context of previously published studies on dietary specializm. The site specificity of demersal fishes has implications for the siting process for marine protected areas as fish species with a strong habitat affinity can be expected to benefit more from such management schemes.

Using knowledge from fishers and fisheries scientists to identify possible groundfish ‘Essential Fish Habitats’

Fishers have often complained that standard fisheries survey data do not adequately reflect the grounds targeted by commercial fishers and hence scientists tend to make over-cautious estimates of fish abundance. Such criticisms are of particular importance if we are to make a creditable attempt to classify ‘Essential Fish Habitat’ using existing large-scale standard trawl surveys. Nevertheless, these data sets provide a powerful tool to examine consistent patterns in the temporal abundance of fish on a spatial scale. Here, we report a questionnaire survey of fishers that invited them to indicate the location of grounds of key importance for gadoid fishes. In addition, fishers were asked to indicate whether they had noticed key habitat features that might indicate the characteristics of EFH. A comparison of such areas as highlighted by fishers with data from standard groundfish surveys were broadly compatible for all three species of gadoids examined. Many sampling stations of these surveys fell outside areas highlighted by fishers as key fishing grounds/habitats. Fishers were able to provide usable biological observations that were consistently cross-referenced by several independent sources, for example the occurrence of haddock over brittlestar beds. We conclude that fishers’ knowledge is an invaluable supplement to existing data sets that can help to better focus more detailed studies of EFH.

Assessment of golden redfish (<i>Sebastes marinus</i> L) in Icelandic waters

Assessment of redfish has traditionally suffered from the inability to age the fish. For golden redfish in Icelandic waters, length distributions and abundance indices are available both from commercial catch and from surveys, particularly a groundfish survey in March. This paper addresses the use of an age- and length-based cohort model for the assessment of redfish, where all selection curves depend on the length of the fish and information on age is not a prerequisite but can be utilised if available. In the last few years it appears that ageing of this species has been successful and in this paper we compare assessment results of a model that incorporates age information with those that do not. Finally, we calculate yield per recruit, Fmax and F0.1 for the stock. Stock size, catch and fishing mortality are then simulated 10 years ahead using different catch control laws.

Variability in maturity and growth in a heavily exploited stock: whiting ( Merlangius merlangus L.) in the Irish Sea

This paper examines the relationships between maturity, length and age of whiting sampled on a length-stratified basis from groundfish surveys of the Irish Sea during spawning in spring 1992–2001. Maturity, defined by the triggering of vitellogenesis or milt production, was a function of both length and age. Proportions of mature individuals in 1-year-old males increased successively from almost zero in length classes below 15 cm to around 0.9 at 25 cm, whilst almost all 2-year-old males were mature from their smallest length of around 19 cm. Maturity in females was more strongly linked to age than to length. Most 1-year-old females were immature, the proportion of mature individuals not exceeding 0.3 in any length class. Most 2-year-old females were mature and immature fish were found in the smallest length classes only (20–25 cm). Almost all 3-year-olds of both sexes were mature in all length classes. Proportions of mature individuals in 1-year-olds increased substantially after 1997, particularly in males. Significant positive cross-correlation between proportion mature and mean length was found for 1-year-olds of both sexes. Length at 50% maturity ( L 50 ) averaged around 19 cm in males and 22 cm in females. Variability in L 50 was negatively cross-correlated with average sea surface temperature in the preceding year. There is no evidence for substantial changes in maturity of whiting since the 1950s, despite an order-of-magnitude reduction in biomass caused by high fishing mortality. Concomitant decreases in mean length-at-age and weight-at-age in recent decades indicate that conditions may have been unfavourable for compensatory changes in maturation.

Length and sex‐specific associations between spiny dogfish (Squalus acanthias) and hydrographic variables in the Bay of Fundy and Scotian Shelf

The associations between spiny dogfish (Squalus acanthias) and hydrographic variables (temperature, salinity and depth) were examined in the Bay of Fundy and Scotian Shelf from 1970 to 1998. Data were obtained from standard groundfish bottom trawl surveys. Dogfish sex affected habitat associations. Males were found to occupy bottom water of significantly higher salinities and depths than that occupied by females. Length also significantly affected habitat associations. Smaller dogfish occupied relatively deep, high salinity bottom water compared with larger dogfish. Overall, the occupied temperatures, salinities and depths were significantly different from those which were available. Dogfish occupied warmer temperatures along a narrow range (6.62–9.19°C) compared with those which were available (1.57–9.35°C). Occupied salinity (32.70–34.43 ppt) and occupied depth (88.62–184.66 m) were also distributed along a narrower range than available salinity (32.16–34.79 ppt) and available depth (55.00–218.10 m). Sex‐specific, length‐specific and overall environmental preference by dogfish may bias traditional `offshore' groundfish surveys while large scale changes in hydrographic parameters may alter dogfish distribution and their interactions with other marine fauna.

Environmental and stock effects on spatial distribution and abundance of mature cod Gadus morhua

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 229:245-262 (2002) - doi:10.3354/meps229245 Environmental and stock effects on spatial distribution and abundance of mature cod Gadus morhua Gavin A. Begg*, Gudrun Marteinsdottir Marine Research Institute, Skulagata 4, PO Box 1390, 121 Reykjavik, Iceland *Present address: CRC Reef Research Centre James Cook University, Townsville, Queensland 4811, Australia. E-mail: gavin.begg@jcu.edu.au ABSTRACT: Identification of environmental and stock effects that influence the spatial distribution and abundance of a spawning stock is necessary for understanding stock and recruitment processes, but data on such effects are rare. We examined environmental (depth, temperature), locational (latitude, longitude), and stock (length, age) effects on the distribution and abundance of mature and spawning cod Gadus morhua in Icelandic waters. Generalized additive models (GAMs) were used to test the effects of these covariates on data collected from historic groundfish surveys (1985 to 1999) around the onset of spawning. Mature cod were present in waters all around Iceland, with persistent concentrations in any given year (indicative of regional spawning components) off the southwest, southeast, and north coasts. Depth, temperature and, more importantly, location were significant covariates affecting both the relative abundance of mature cod and the spatial distribution of spawning cod. Incorporating the effects of these covariates and their underlying relationships into stock and recruitment processes will provide insight into the mechanisms involved in population structuring of cod in Icelandic waters. KEY WORDS: Stock · Recruitment · Environment · Generalized additive models · Population dynamics Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 229. Online publication date: March 20, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

Spatial patterns of groundfish assemblages on the continental shelf of Portugal

The analysis of catch data from groundfish surveys (1985–1988) conducted on the Portuguese shelf and upper slope (20–500 m) revealed five major geographic areas, each characterized by the presence of a typical fish assemblage. These areas of distinct fish assemblages were found to be closely aligned with depth, in a way that resembles spatial patterns previously described for other continental shelves in the North Atlantic. The sharpest biological transition on the Portuguese shelf takes place as one moves from areas shallower than 120 m (“Shallow Groups”) towards deeper locations offshore (“Deep Groups”). Beyond the 150 m isobath, the biomass was dominated by blue whiting, whereas inshore variability in assemblage composition was much greater. Species such as sardine, horse mackerel, mackerel (to the north of Lisbon) and sparids (to the south) comprised significant and highly variable proportions of the population abundance inshore. There are similarities between the trophic and spatial organization of the marine community on the Portuguese shelf and that of other coastal upwelling ecosystems that are briefly reviewed here.

The structure of demersal assemblages off Namibia in relation to abiotic factors

Groundfish surveys carried out between October 1992 and October 1996 were used to describe the demersal assemblages of the northern Benguela along the Namibian coast. Multivariate analyses indicated a clear separation between shelf and slope habitats, which were divided into three and two assemblages respectively. These divisions were distinct, with clear distributional boundaries separating them. Species determining the structure of these divisions and subdivisions and their relative abundances were identified. Depth, bottom temperature, bottom salinity and dissolved oxygen were all significantly correlated with the spatial distribution of the assemblages. Minor changes or shifts in the distribution of assemblages were observed over the study period, although patterns of species associations remained relatively constant. Keywords: community, demersal, environment, fishery, Namibia, trawling African Journal of Marine Science 2001, 23: 397–417

The association between arm damage of the common starfish,Asterias rubens, and fishing intensity determined from aerial observation

Abstract The use of the common starfish Asterias rubens as a biological indicator of physical disturbance was investigated during a two year period at 32 offshore sampling sites distributed over a wide geographical area. Starfish were collected from the Irish Sea and Bristol Channel during routine groundfish surveys, and the proportion of each starfish sample with missing or partly re-grown arms was recorded. Significant positive correlations were found between the proportion of damaged starfish, and the intensity of bottom-trawl activity at the sample sites observed during the previous 2–8 months by aerial reconnaissance flights. The degree of variability in these relationships, however, suggested thar ann damage of starfish can be used only as a broad-scale indicator of the intensity of commercial fishing activity. The implications of this for interpreting changes in benthic community structure are discussed.

Growth, sexual maturation and spawning in central North Sea plaice ( Pleuronectes platessa L.), and the generation of maturity ogives from commercial catch data

This paper investigates sexual maturation in North Sea plaice using data from the commercial catch of plaice landed on Lowestoft fish market from 1983–1996, along with data from the English summer groundfish surveys from 1977–1993. Two offshore spawning areas were identified in the central North Sea – central-northern and central-southern. A general linear model (GLM) showed plaice sexual maturation was influenced by body weight, age and geographical location and was subject to annual and seasonal variation. Few three year old female place appeared to spawn in the central North Sea, similar to the situation early in the 20th century. The condition factor of plaice peaked at the start of spawning and declined sharply during spawning, with older females investing the most heavily in reproduction, both in absolute and relative terms. The loss of older females through fishing is, therefore, likely to compromise the overall reproductive fitness of the remaining population, comprising of young females. The reproductive fitness of the males was also compromised, since young males tended to be in running condition for about a month less than older males. Reproductive investment in females was about twice the level found in males. There was evidence that plaice in the northern North Sea matured at a slower rate than fish in the south. From the mid-1980s until the mid-1990s there was an increase in the proportion of young plaice reaching sexual maturity, following an earlier decline from the late 1970s. Problems with using maturity ogives derived from market sampling data in order to assess the proportion of spawning plaice in the North Sea are discussed.

Biology and ecology of Nezumia liolepis and N. stelgidolepis from the west coast of North America

Nezumia liolepis and Nezumia stelgidolepis were collected during a National Marine Fisheries Service 1997 groundfish survey along the Pacific coast of North America. Bottom trawling was conducted from 188 to 1260 m depths. Nezumia liolepis were collected from 581 to 1247 m (3·4–5·7°C) and N. stelgidolepis from 285 to 555 m (6·0–8·4°C). The two species had distinct depth and temperature distributions and the majority of the specimens for each species came from narrow depth ranges. Nezumia stelgidolepis attains a larger pre‐anal fin length (117 mm) than N. liolepis (83 mm), and age estimates from otolith ring counts indicate ages from 3 to 9 years for N. liolepis and 7 and 13 years for N. stelgidolepis from specimens collected. The gonads of N. liolepis were not reproductively active, and a single N. stelgidolepis possessed eggs of various sizes suggesting batch spawning. N. liolepis and N. stelgidolepis fed benthically, primarily on crustaceans such as amphipods, shrimp, mysids and polychaete worms. Although diet overlap was high, these two species appear to limit competition for resources by habitat separation.

Growth and Maturation of Haddock (Melanogrammus aeglefinus) in Icelandic Waters

Growth and maturation of haddock (Melanogrammus aeglefinus), in Icelandic waters have shown considerable spatial and temporal variation in the past several decades. After 1988 haddock have matured much earlier than in earlier decades and haddock in some areas appear to mature young and also show stunted growth. The paper compares possible reasons for this phenomenon and evaluates different models of the growth-maturation process through the use of otolith radii and length-at-age as well as biological samples from groundfish surveys and commercial catches. It is found that there is considerable spatial and temporal variation in growth and maturity. Some of the variability may be due to some form of competition among year-classes whereas there are also indications of a link between growth and maturation in certain areas.

Feeding habits of John Dory, Zeus faber, off the Portuguese continental coast

The feeding habits of John Dory (Zeus faber) were studied, based on the analysis of stomach contents from fish sampled in five groundfish surveys. These surveys were carried out off the Portuguese coast during different seasons between 1990 and 1992. The main aspects of feeding biology analysed in this paper are: ontogenetic diet changes, temporal variations in food composition and feeding intensity. Multivariate methods were used to investigate ontogenetic diet shifts. Two main length groups were identified: 8.0–24.9 cm fish, feeding mainly on dragonets and silvery pout, and 25.0–55.9 cm fish whose diet was mainly composed of blue whiting and snipefish. A transitional phase (24.0–30.9 cm fish) with a mixed food composition was observed. This ontogenetic diet shift does not seem to correspond to any important change in body morphology but it does coincide with the onset of sexual maturity in the species.John Dory switched from a diet of small prey species with more pronounced benthic behaviour to a diet of larger schooling pelagic species. This suggests parallel evolution to more pelagic foraging behaviour. However, John Dory feeding habits appear to be largely controlled by the availability and accessibility of prey species: (i) the diet of adult John Dory is dominated by very abundant species; (ii) shifts in the main prey items between different times of the year and between different areas seem to be related both with their absolute and relative abundance in the environment and with the overlap between the depth distribution of predator and prey.

Fishing effects in northeast Atlantic shelf seas: patterns in fishing effort, diversity and community structure. VI. Gale effects on vertical distribution and structure of a fish assemblage in the North Sea

The short-term effects of a gale on diversity and assemblage patterns of groundfish in a defined area in the German Bight were analysed using univariate and multivariate statistical techniques. After a gale of considerable strength, the catch rates of some groundfish species exhibited significant daily changes. Catches of the flatfishes dab ( Limanda limanda), solenette ( Buglossidium luteum), plaice ( Pleuronectes platessa) and sole ( Solea vulgaris) changed markedly between the first and the second day after the gale. In general, catches of flatfishes were significantly higher on the first day after the gale than before or after, whereas catches of the pelagic species scad ( Trachurus trachurus) and mackerel ( Scomber scombrus) were significantly higher on the second day after the gale. The detection of these changes in the species assemblage was supported by shifts in diversity indices and multivariate description of community structure (cluster analysis, MDS and SIMPER analysis). The observed differences could be explained by species-specific reactions to higher turbidity and bottom oscillation currents during increased wave-action in shallower waters. The results have important implications for those concerned with the analysis and interpretation of fish abundance data collected during groundfish surveys.

Biodiversity and abundance of the zooplankton of the Northeast Shelf ecosystem

Since 1961, the Northeast Fisheries Science Center of the National Marine Fisheries Service has been conducting a study of the zooplankton of the US Northeast Shelf ecosystem. One of the objectives is to investigate ecological patterns of the system for any evidence of regime shifts and their effects on biodiversity and abundance of zooplankton and fish stocks. Two sampling strategies have been used: collections with Continuous Plankton Recorders beginning in 1961; and collections with bongo nets during the decade of MARMAP surveys (1977-1987), augmented by a time series of collections made during the autumn and spring groundfish surveys (1977-1993). The climate regime of the region leads to distinct and recurring seasonal pulses in zooplankton biomass and species succession in each of the four subareas of the shelf ecosystem distinguished. Evidence was detected in the analyses of inter-annual variability in species abundance and zooplankton biomass. However, no evidence of a large-scale regime shift of the magnitude described recently for other areas was detected from our analyses. Species shifts in abundance and the inter-annual and decadal variability observed in zooplankton biomass levels have allowed for sufficient residual sustainability in zooplankton biodiversity and abundance to support the recovery of depleted pelagic fish stocks.

Method for estimating variability due to sampling of catches on a trawl survey

Many catches arising on trawl surveys of fish stocks are sampled for the measurement of fish lengths and the collection of otoliths for age determination, thereby increasing the variability of the estimated abundance indices, expressed as catch per hour by species and year-class. This paper demonstrates how catch-sampling variability can be assessed, using the English groundfish survey of the North Sea (EGFS) as an example for analysis. Unbiased estimators are derived for catch-sampling covariance matrices for numbers-at-length of a species by size category, by haul, and by sampling stratum and for numbers-at-age by stratum. The same statistical principles could be applied to derive appropriate estimators for other surveys having different catch-sampling procedures. A method is also put forward for estimating the relative importance of variability due to catch sampling and that due to the design of the survey, i.e., due to the number, location, and timing of fishing stations and the method of fishing. Analysis of a small set of EGFS data for haddock (Melanogrammus aeglefinus) is included for illustrative purposes. Implications for survey design and catch-sampling efficiency are briefly discussed.

Intercalibration of age readings of ling (Molva molvaL.) blue ling (Molva dipterygiaPennant, 1784) and tusk (Brosme brosmeL.)

To facilitate intercalibration of the age readings of ling, blue ling and tusk, series of otoliths were exchanged among the fisheries laboratories in Iceland, the Faroes and Norway. For ling and tusk, the precision was significantly improved by the exchange exercises, but remained relatively low. Many large specimens could not be aged with confidence, and for tusk of all sizes a rather high proportion of the otoliths were unreadable. For blue ling, some progress was made using otolith sections of juvenile fish, but it remained difficult to decide what patterns should be accepted as annuli. The age readings from otoliths of 1-2 group tusk and blue ling corresponded well with assumed ages based on modes in the length-frequency distributions from Icelandic groundfish surveys.