Total Stock Size Research Articles

Predator-prey collapses at the edge of predator distribution: the case of clupeids and common guillemots (Uria aalge) in NW Iberia

The spatial structuring of seabird populations makes individual colonies very dependent on regional factors. That is especially the case in small edge populations located far from large colonies. We analysed retrospectively the poorly known collapse, some 50 years ago (around 1962-1973), of a relatively small population of breeding common guillemots (Uria aalge) located at their southernmost limit of distribution in Europe (NW Iberia). We assumed that guillemots behaved locally as facultative specialists in small pelagic fish due to the occurrence of a strong clupeid fishery, and we studied the association between the guillemot collapse and annual regional landings of sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus), used as a proxy of total stock size. The overall relationship between ln-transformed guillemot counts (May-June) and ln-transformed sardine landings (May-October) was stronger than the fit on untransformed variables (r2 =0.52 vs. 0.27), indicating an exponential relationship between the non-transformed variables. This relationship was somewhat stronger and linear after the collapse, when only a few tens of guillemots remaining (r2 =0.60). A strong regime shift in sardine landings was detected in 1968 and also in anchovy landings in 1969. The overall relationship between guillemot numbers and anchovy landings was linear and strong (r2 =0.72) but completely dependent on the large 1960s estimate of guillemots. However, no relationship was found between guillemot numbers and anchovy landings (April-June) after the guillemot collapse. The most likely period for the guillemot collapse was therefore 1968-1970, as seabird colonies are known to collapse immediately after their staple prey crashes. Local guillemot colonies were not subsequently rescued by immigration and have remained empty or almost empty until present, showing the high sensitivity of edge populations to environmental variability at the regional scale.

Yellow eel (Anguilla anguilla) density trends along the German part of the southern Baltic Sea between 2009 and 2020

According to the recruitment indices provided by ICES for the years 2011–2021, the natural recruitment of the European eel remains at low level. However, fishery-dependent data series indicate an increasing trend in the southern Baltic Sea within recent years. In the light of the pan-European conservation efforts, the question is whether fishery-independent monitoring approaches can verify this positive trend. To monitor yellow eel density, a 1 ha enclosure system for non-tidal coastal waters was used in eight reference areas along the German part of the southern Baltic Sea (Federal State Mecklenburg-Western Pomerania) from 2009 to 2020. Changes in presence rate, mean yellow eel density and total length distribution were investigated. Additionally, the total stock size of yellow eels and the annual settlement rate of juvenile eels were estimated. Overall, the enclosure data indicate a positive trend in yellow eel stock size in recent years. An increasing mean yellow eel density was observed in five out of eight reference areas sampled. The density change was linked to an increased number of juvenile eels as evidenced by a significant change in the total length distribution. The mean annual settlement rate of juvenile eels was estimated as 13.2 eels of age class 0 + ha−1 year−1 in 2012–2017. Besides the increased settlement rate, other mechanisms of population dynamics or conservation measures might have contributed the observed positive trend in yellow eel densities.

Estimation of In‐Water Density and Abundance of Harbor Seals

ABSTRACTEcologists and managers require accurate population estimates of marine mammals to assess potential anthropogenic threats to these animals. We present estimates of in‐water density and abundance of a distinct stock of harbor seals (Phoca vitulina richardii) in Hood Canal, Washington, USA. We used aerial line‐transect survey data collected from 2013 to 2016 to directly estimate harbor seal density and abundance in the waters of Hood Canal, a deep‐water fjord in the Salish Sea. We estimated a correction factor for trackline detection probability from dive and surface time data gathered from regional seal tagging studies, and applied this factor to correct for seals missed on the trackline during surveys. We applied conventional and multiple covariate line‐transect approaches in the analysis. The resulting best estimate of in‐water density of harbor seals in the Hood Canal study region was 5.80 seals/km2, with an estimated abundance of 2,009 seals. We did not derive a correction factor to account for the number of seals on land (i.e., hauled out). Therefore, these estimates do not reflect total stock size but provide a starting point to evaluate potential influences of anthropogenic activities, particularly those involving underwater noise, on this marine mammal stock. © 2021 The Wildlife Society.

Spatial variability of Spanish sardine (Sardinella aurita) abundance as related to the upwelling cycle off the southeastern Caribbean Sea.

The Sardinella aurita fishery off northeastern Venezuela, region of seasonal wind-driven coastal-upwelling, accounts for 90% of the Caribbean Sea small pelagic catch. This law-protected artisanal fishery takes place up to ~10 km offshore. The spatial distribution, number of schools, and biomass of S. aurita were studied using eight hydro-acoustic surveys (1995–1998). The study included the analysis of satellite-derived sea surface temperature and chlorophyll-a. Surveys were grouped by strong, weak, and transitional upwelling seasons. Relationships between these observations were analyzed using Generalized Additive Models. Results show that during the primary upwelling season (January-May) sardines were widely distributed in upwelling plumes that extended up to 70 km offshore. In the other hand, during the weak upwelling season (September-October) higher sardine densities were found within 10 Km off the coastal upwelling foci. The number of small pelagic schools was directly correlated with small pelagic densities; however, regardless of the season, higher numbers of small pelagic schools were always closer to the shoreline, especially during warm conditions. These two behaviors increase the availability and catchability of sardines for the artisanal fishery during the warm season, regardless of the total stock size. Using this evidence, we pose the hypothesis that the collapse of the regional S. aurita fishery in 2005 was due to a combination of stressful habitat conditions sustained since 2004. These included bottom-up factors due to food scarcity caused by weak upwelling, combined with top-down stress due to overfishing, as sardines accumulated in narrow diminished upwelling plumes located close to the coast. The increased catchability within easily accessible upwelling foci led to the demise of this biological resource, which as of 2014 had not yet recovered. Environmental conditions affecting the sardine habitat needs to be taken into account for the management of this stock. For example, during years with weak upwelling, special measures should be taken during the warm season on the second half of the year to avoid further pressure on the stock.

High-seas fish wars generate marine reserves

The effective management of marine fisheries is an ongoing challenge at the intersection of biology, economics, and policy. One way in which fish stocks-and their habitats-can be protected is through the establishment of marine reserves, areas that are closed to fishing. Although the potential economic benefits of such reserves have been shown for single-owner fisheries, their implementation quickly becomes complicated when more than one noncooperating harvester is involved in fishery management, which is the case on the high seas. How do multiple self-interested actors distribute their fishing effort to maximize their individual economic gains in the presence of others? Here, we use a game theoretic model to compare the effort distributions of multiple noncooperating harvesters with the effort distributions in the benchmark sole owner and open access cases. In addition to comparing aggregate rent, stock size, and fishing effort, we focus on the occurrence, size, and location of marine reserves. We show that marine reserves are a component of many noncooperative Cournot-Nash equilibria. Furthermore, as the number of harvesters increases, (i) both total unfished area and the size of binding reserves (those that actually constrain behavior) may increase, although the latter eventually asymptotically decreases; (ii) total rents and stock size both decline; and (iii) aggregate effort used (i.e., employment) can either increase or decrease, perhaps nonmonotonically.

Changes in the Norwegian breeding population of European shag correlate with forage fish and climate

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 489:235-244 (2013) - DOI: https://doi.org/10.3354/meps10440 Changes in the Norwegian breeding population of European shag correlate with forage fish and climate Jan Ove Bustnes1,*, Tycho Anker-Nilssen2, Kjell Einar Erikstad1,3, Svein-Håkon Lorentsen2, Geir Helge Systad1 1Norwegian Institute for Nature Research, FRAM - High North Research Centre on Climate and the Environment, 9296 Tromsø, Norway 2Norwegian Institute for Nature Research, 7485 Trondheim, Norway 3Centre for Biodiversity Dynamics (CBD), Dept of Biology, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway *Email: jan.o.bustnes@nina.no ABSTRACT: While many seabird species in the North Atlantic have declined over the last decades, the Norwegian population of the European shag Phalacrocorax aristotelis has increased. In the present study, we assessed the impact of food availability and climate on the shag population by analysing 25 years of data (1985 to 2009) on breeding numbers in 3 large colonies: 2 in the Norwegian Sea (65°N and 67°N) and 1 in the Barents Sea (70°N). Predictor variables were ICES abundance estimates of young saithe Pollacius virens, the most important forage fish for shags in the Norwegian Sea, and for the Barents Sea colony also total stock size estimates of Barents Sea capelin Mallotus villosus. As proxies for climate variation, we used the North Atlantic Oscilliation index (NAO) for the last and the preceding winter (lagged by 1 yr). Finally, the annual population size of the study colonies in the preceding year was included in the models to control for potential density-dependent effects. The predictor variables explained 46 to 67% of the variation in annual growth rate in the colonies. In the Barents Sea colony, the shag population growth rate was only associated with capelin abundance, whereas for the Norwegian Sea colonies, there were strong positive relationships with 1 yr old saithe and a negative effect of the lagged NAO winter index. The latter effect may be a result of unfavourable weather conditions with high winds and precipitation in winter increasing mortality among non-breeding age classes of shag. Our study is the first to demonstrate a close correlation between stock estimates of the primary forage fish for European shags and shag breeding numbers. This suggests that the population growth rate and diet of shags may be used as cost-efficient and reliable indicators of major shifts in saithe stock recruitment. KEY WORDS: Phalacrocorax aristotelis · Norwegian Sea · Barents Sea · NAO · Saithe · Breeding numbers Full text in pdf format Supplementary material PreviousNextCite this article as: Bustnes JO, Anker-Nilssen T, Erikstad KE, Lorentsen SH, Systad GH (2013) Changes in the Norwegian breeding population of European shag correlate with forage fish and climate. Mar Ecol Prog Ser 489:235-244. https://doi.org/10.3354/meps10440 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 489. Online publication date: August 28, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Spatial and temporal distribution of Atlantic mackerel (Scomber scombrus) along the northeast coast of the United States, 1985–1999

Abstract Radlinski, M. K., Sundermeyer, M. A., Bisagni, J. J., and Cadrin, S. X. 2013. Spatial and temporal distribution of Atlantic mackerel (Scomber scombrus) along the northeast coast of the United States, 1985–1999. – ICES Journal of Marine Science, 70: 1151–1161. The distribution of Atlantic mackerel (Scomber scombrus) during their spring migration along the Mid-Atlantic Bight and into the Gulf of Maine has historically been associated with spring warming along the continental shelf. Variations in mackerel distributions based on National Marine Fisheries Service spring surveys were compared with variations in sea surface temperature (SST) from satellite remote sensing for the eastern US continental shelf for the period 1985–1999. The mackerel stock was first analysed as a unit, then separated into three size classes to assess differences in distribution among years and individuals of various lengths. Results showed an across-shelf correlation between catch and March SST in the Mid-Atlantic Bight for both the entire population and each size class. Along-shelf catch variations were correlated with SST for large mackerel, but not total stock or smaller size classes. Finally, the distribution of mackerel length in the Gulf of Maine was negatively correlated with March SST in the Great South Channel. Results suggest surface temperature along the northeast continental shelf may be used to predict certain, but not all, aspects of annual migration along the shelf, and that factors in addition to temperature are also important in controlling distributions of Atlantic mackerel.

Total stock size and cannibalism regulate recruitment in cod (Gadus morhua) on the Faroe Plateau

Abstract Steingrund, P., Mouritsen, R., Reinert, J., Gaard, E., and Hátún, H. 2010. Total stock size and cannibalism regulate recruitment in cod (Gadus morhua) on the Faroe Plateau. – ICES Journal of Marine Science, 67: 111–124. Year-class strength of fish is often considered to be determined at the pelagic larval stage, but we show that year-class strength of cod on the Faroe Plateau seems to be determined later, at the 1- or 2-group stage. Adult cod (C), measured in terms of the catch per unit effort (cpue) of small longliners, move into nearshore nursery areas of juvenile cod when in poor condition and probably displace 1-year-old cod to deeper water, where they are cannibalized. In addition, the recruitment of 2-year-old cod, at least up to a certain level, is positively related to the total biomass (B) of older cod on the Faroe Plateau, which are present at about the same time as the recruitment event. This feature, which seems to be a new observation in terms of the recruitment dynamics of cod, is possibly related to enhanced foraging or a reduced predation risk. The recruitment of 2-year-old cod during the years 1984–2006 is described by aB/C + d (r2 = 0.87), where a and d are fitted constants. The implications for stock assessment and management are discussed.

Response in stock size and recruitment of northern shrimp (Pandalus borealis) to changes in predator biomass and distribution in west Greenland waters

Potential relationships between stock size of Northern shrimp (Pandalus borealis) and its main predators, Atlantic cod (Gadus morhua) and Greenland halibut (Reinhardtius hippoglossoides), in West Greenland waters were examined. Changes in spatial overlap between the species were taken into account in the analysis of the impact of predator biomass on the survival of Northern shrimp recruits. Survey estimates of total stock size of Northern shrimp were negatively correlated with Atlantic cod biomass lagged by one and two years until the beginning of the 1990s. Thereafter, the Atlantic cod stock in West Greenland offshore waters collapsed, but no immediate response of Northern shrimp to the released predation pressure was observed. Recruitment and subsequently stock biomass of Northern shrimp increased first about 10 years later following a moderate increase in the temperature conditions, which had likely enhanced plankton productivity. After a record high level in 2000, survival of juvenile Northern shrimp, i.e. the number of recruits at age 2 per unit of spawning stock biomass, decreased drastically. Statistical analysis suggests that increased predation by Greenland halibut have overruled the positive effect of favourable temperature conditions in the most recent years. In contrast, the impact of Atlantic cod on Northern shrimp was insignificant in the recent years due to low Atlantic cod biomass and a very limited spatial overlap between the two species.

Stock–Yield Model for a Fish with Variable Annual Recruitment

AbstractMany fish populations have variable annual recruitment, which challenges both population model development and conventional management strategies. Here, we develop a stock– yield model to assess the sustainability of the commercial catch of goldeye Hiodon alosoides from western Lake Athabasca of northeastern Alberta, Canada. Goldeyes have highly irregular annual recruitment; many year‐classes are poor and dominant year‐classes appear an average of only 1.7 times per decade. In the late 1990s, the mean annual commercial catch from this population was about 11,500 goldeyes—primarily fish from the 1982 and 1989 year‐classes. A best‐fit stock–yield model developed for this population incorporated (1) the restrictions of the catch from a historic fishery that depleted this population during the 1950s and early 1960s; (2) the restrictions of the known total stock size in the early 1970s, obtained from mark–recapture studies; (3) long‐term annual estimates and patterns for stock recruitment (1972–2008); (4) a natural annual mortality rate of 20%; and (5) stock estimates for 1994 obtained from the catchability coefficient (catch per unit effort divided by the magnitude of commercial stock estimated from mark–recapture data for 1972 and 1973). The model suggested that an annual commercial catch of 10,000 goldeyes was sustainable over all years and conditions (1947–2004). This small modeled catch, about 2% of the long‐term mean stock number of 540,000 goldeyes, ensured that at least a few thousand mature goldeyes would survive to spawn through periods of poor stock recruitment lasting more than a decade. These results suggest that fish populations with irregular annual recruitment can sustain only a low exploitation rate to prevent stock extinction during long periods of low recruitment.

Spatial and temporal trends in abundance of Atlantic salmon, Salmo salar, in Newfoundland with emphasis on impacts of the 1992 closure of the commercial fishery

AbstractClosure of the Newfoundland commercial Atlantic salmon, Salmo salar L., fishery in 1992 was the most restrictive measure introduced to help rebuild depressed local stocks of salmon. Here, the effects of the closure are evaluated by analysing trends in abundance since 1984, and estimates of survival in both freshwater and marine environments derived from enumeration of salmon at fish counting facilities. While freshwater production of smolts generally has been maintained, marine survival rates remain low (2–10%), and highly variable. Overall, total stock size differs little from that prior to the closure of the commercial salmon fishery. Spawning escapements have increased by a factor of 2 or 3 in some rivers, but in other areas total returns are lower on average than those prior to the fishery closure. Factors other than exploitation are contributing to lack of stock recovery, resulting in continued conservation concerns.

Food consumption estimates of Barents Sea harp seals

The consumption of various prey species, required by the Barents Sea harp seal (Phoca groenlandica) stock in order to cover their energy demands, has been estimated by combining data on the energy density of prey species and on seasonal variations in the energy expenditure and body condition of the seals. Data on diet composition and body condition were collected in the period 1990-1996 by sampling harp seals during different seasons, in various areas of the Barents Sea. All diet composition data were based on reconstructed prey biomass, and adjustments were made for differences in digestibility of crustaceans and fish. The number of seals representing different age and sex groups were calculated for the entire population, and the monthly food requirements were estimated. In 1998, the total Barents Sea harp seal stock was estimated to comprise 2.22 million seals based on a mean production of 301,000 pups. After adjustments for a pup mortality of 30% its total annual food consumption was estimated to be in the range of 3.35-5.05 million tonnes (depending on choice of input parameters). Assuming that there are seasonal changes in basal metabolic rate associated withchanges in body mass, and that the field metabolic rate of the seals corresponded to two times their predicted basal metabolic rate, the annual food consumption of the Barents Sea harp seal stock was estimated. If capelin (Mallolus villosus) was assumed to be abundant, the annual total consumption was estimated to be 3.35 million tonnes, of which 1,223,800 tonnes were crustaceans, 807,800 tonneswere capelin, 605,300 tonnes were polar cod (Boreogadus saida), 212,400 tonnes were herring (Clupea harengus), 100,500 tonnes were cod (Gadus morhua) and 404,200 tonnes were "other fish". A very low capelin stock in the Barents Sea (as it was in the period 1993-1996) led to switches in seal diet composition, with increased consumption of polar cod (from ca. 16%-18 % to ca. 23%-25 % oftotal consumption), other gadoids (dominated by cod, but also including haddock (Melanogrammus aeglefinus) and saithe (Pollachius virens)), herring, and "other fish". Using the same set of assumptions as in the previous estimate, the total consumption would have been 3.47 million tonnes, divided between various prey species as follows (in tonnes): polar cod 876,000, codfish (cod, saithe and haddock) 359,700, "other fish" 618,800, herring 392,500, and crustaceans 1,204,200. Overall, the largest quantities of food were estimated to be consumed in the period June-September.In 1999, the total Barents Sea harp seal stock size was estimated to be 2.18 (95% CI, 1.79 to 2.58) million animals, which would give an annual food consumption in the range of 2,69 - 3.96 million tonnes (based on upper and lower 95% confidence limits and adjusted for a pup mortality rate of 0.3) if capelin is assumed to be abundant.

Mapping the abundance of riverine fish populations: integrating hierarchical Bayesian models with a geographic information system (GIS)

A hierarchical Bayesian model is described for mapping the abundance of fish throughout a watershed from single- and multiple-pass removal sampling. A geographic information system (GIS) was used to generate a raster-based model of the river network, which provided three benefits for estimating fish density. Firstly, the horizontal resolution of the raster (50 m) provided an approximation to the statistical sampling frame and allowed correction for the finite number of potential sampling sites in a reach. Secondly, the modelled river network generated explanatory variables for every site in the network, facilitating the mapping of predicted densities and providing the basis for stratified or regression estimators for reach-specific densities. Finally, the spatial autocorrelation of fish densities was modelled in terms of the distance along the river network. A similar Bayesian model was also developed for the wetted width of the river network, and this was combined with the density model to provide estimates of the total stock size. The model was implemented using Markov chain Monte Carlo simulation in the statistical package WinBUGS. Application of the hierarchical model is illustrated with electrofishing data for age-1+ sea trout (Salmo trutta) in a small tributary of the River Conwy, Wales.

Subsistence utilization of the crab Neosarmatium meinerti in the Kosi Lakes ecosystem, KwaZulu-Natal, South Africa

Members of the rural communities living around the edges of the Kosi Lake system in northern KwaZulu Natal have been subsistence users of the resources of the lakes, estuary and surrounding swamp forest for many generations. Women and children dig up a variety of crabs in the mangrove ecosystem, and in the late 1980s collected as many as 638 000 crabs a year. The red mangrove crab Neosarmatium meinerti de Man, 1884 (Crustacea: Decapoda: Grapsidae: Sesarminae) constituted the major proportion of these catches, while Scylla serrata and Cardisoma carnifex made up the rest. However, subsistence fishing effort and catches have been reduced steadily, declining from about 5000outings p.a. and between 2 and4×105 crabs p.a. in the 1980s, to about 1000 outings and 1.3 × 105 crabs in the late 1990s. CPUE has increased from 66 to 145 crabs/person/outing during this period. Crab densities ranged between 0.35/m2 on the high shore and 11.4/m2 on a densely vegetated island, but it was not clear what factors most affected crab density. Densities inside and outside a ‘sanctuary area’ were not significantly different, although there were significant differences in densities among sites within the ‘sanctuary area’. Stock size within the entire mangrove area of Kosi was estimated as 728 514 N. meinerti, butthis is almost certainly an underestimate of the total stock size. Sex ratios in the N. meinerti population were not significantly different from 1:1 but above 38 mm carapace width (CW) male crabs considerably outnumbered females. Females matured at 24.9 mm CW and mean sizes of crabs captured by subsistence harvesters were 33.4 mm and 31.3 mm CW for males and females, respectively. Instantaneous natural mortality was estimated at M = 2.27 and fishing mortality F = 0.83. The N. meinerti population in the Kosi mangrove system appears to be sustainably exploited.

The use of adaptive cluster sampling for hydroacoustic surveys

Resource managers are often required to estimate the size of a wildlife population based on sampling surveys. This problem is especially critical in fisheries, where stock-size estimation forms the basis for key policy decisions. This study looks at design-based methods for a hydroacoustic fisheries survey, with the goal of improving estimation when the target stock has a patchy spatial distribution. In particular, we examine the efficiency and feasibility of a relatively new design-based method known as adaptive cluster sampling (ACS). A simulation experiment looks at the relative efficiency of ACS and traditional sampling designs in a hydroacoustic survey setting. Fish densities with known spatial covariance are generated and subjected to repeated sampling. The distributions of the different estimators are compared.Hydroacoustic data frequently display strong serial correlation along transects and so traditional designs based on one-stage cluster sampling are appropriate. Estimates of total stock size for these designs had a markedly skewed distribution. ACS designs performed better than traditional designs for all stocks with small-scale spatial correlation in fish density, yielding estimates with lower variance. ACS estimators were not skewed and had a lower frequency of large errors. For the most variable stock the use of ACS reduced the coefficient of variation (CV) of the stock size estimate from over 0.9 to around 0.5. Differences between traditional and ACS designs were consistent over multiple realizations of each spatial covariance model.A survey of rainbow smelt (Osmerus mordax) in the eastern basin of Lake Erie was used as a case study for development of a survey design. A field trial showed that use of ACS for the survey is feasible but pointed out some areas for further research. The biggest drawback to use of ACS is uncertainty in the final sample size. This can be partially controlled by applying ACS within a stratified design. ACS retains the unbiased and non-parametric properties of design-based estimation but allows increased sampling in high-density areas that are of greater biological interest. For stocks with an aggregated or patchy spatial distribution ACS can provide a more precise estimate of stock size than traditional survey methods.

Management implications of interactions between fisheries and sandeel-dependent seabirds and seals in the North Sea

The lesser sandeel, Ammodytes marinus , is a key food for many seabirds and seals, and is also the target of the largest single-species fishery in the North Sea. Despite claims that sandeel fishing has harmed dependent predator populations, census data show that most seabirds and grey seals increased in numbers as the fishery grew and reached peak harvest. Generally high breeding success of black-legged kittiwakes at North Sea colonies also suggests that sandeel abundance has remained good for breeding seabirds at the broad scale, though local and small-scale effects of sandeel fishing should not be overlooked. VPA and CPUE data suggest that abundance increased as the fishery grew. A negative correlation between sandeel recruitment and total stock size preceding spawning suggests that there is now resource competition (bottom-up control). Bioenergetics modelling indicates that predatory fish take far more sandeel than taken by the industrial fishery or wildlife. Effects of decreases in predatory fish stocks have been greater than increases in the take by seabirds and seals and by the fishery. Thus, overall, there appears to have been a reduction in mortality during the last 30 years. Changes in predatory fish abundances, especially mackerel and whiting, may influence sandeel stocks more than changes in industrial fishery, at least at the scale of the North Sea as a whole. These interactions imply that seabird and seal food supply in terms of sandeel may be strongly dependent on decisions regarding management of stocks of mackerel and gadoids. The overwhelming influence of predation on “food-fish” by predatory fish may be a feature of many marine food webs worldwide, where “fishing down the food web” has occurred, and this has clear management implications if wildlife and fisheries are to coexist.

Long‐term indirect effects of mechanical cockle‐dredging on intertidal bivalve stocks in the Wadden Sea

Summary There is world‐wide concern about the effects of bottom‐dredging on benthic communities in soft sediments. In autumn 1988, almost a third of the 50‐km2 intertidal system around the island of Griend in the western Dutch Wadden Sea was suction‐dredged for edible cockles Cerastoderma edule and this study assessed subsequent effects. An adjacent area not directly touched by this fishery and an area from which the mussel Mytilus edulis beds were removed, served as reference areas. Sediment characteristics, together with the total stock size and settlement densities of Cerastoderma, Baltic tellin Macoma balthica and soft‐shelled clam Mya arenaria, were documented during 11 successive autumns before (August–September 1988) and after (August–September 1989–98) the suction‐dredging event in fished and unfished areas. Four other areas in the Dutch Wadden Sea, where changes in densities of juvenile bivalves from 1992 to 1998 were measured, served as additional reference locations. Between 1988 and 1994, median sediment grain size increased while silt was lost from sediments near Griend that were dredged for cockles. The initial sediment characteristics were re‐attained by 1996. After the removal of all Mytilus and most Cerastoderma, the abundance of Macoma declined for 8 years. From 1989 to 1998, stocks of Cerastoderma, Macoma and Mytilus did not recover to the 1988 levels, with the loss of Cerastoderma and Macoma being most pronounced in the area dredged for cockles. Declines of bivalve stocks were caused by particularly low rates of settlement in fished areas until 1996, i.e. 8 years after the dredging. A comparison of settlement in the short (1992–94) and medium term (1996–98) after cockle‐dredging in several fished and unfished areas spread over the entire Dutch Wadden Sea, showed a significant negative effect of dredging on subsequent settlement of Cerastoderma. Macoma also declined, but not significantly. We conclude that suction‐dredging of Cerastoderma had long‐lasting negative effects on recruitment of bivalves, particularly the target species, in sandy parts of the Wadden Sea basin. Initially, sediment reworking by suction‐dredging (especially during autumn storms) probably caused losses of fine silts. Negative feedback processes appeared to follow that prevented the accumulation of fine‐grained sediments conducive to bivalve settlement.

Distribution and Numbers of Bowhead Whales (<i>Balaena mysticetus</i>) in Northwestern Hudson Bay in August 1995

There is interest among the Inuit of Nunavut in renewing subsistence hunting of bowhead whales. Managing a limited harvest while allowing for stock recovery from commercial whaling requires some estimate of stock numbers. The large geographic range of bowhead whales in the eastern Canadian Arctic precludes cost-effective estimation of total stock size; however, estimates of summer aggregation sizes can be derived from sampling of summering areas. Although these numbers do not represent total stock size, they do provide indicators of the minimum number of bowheads known to be present and may be useful as indices for monitoring stock recovery. A visual aerial survey conducted in northwestern Hudson Bay resulted in an estimate of 75 ± 27.5 (95% Confidence Interval = 17-133) bowhead whales. This estimate is conservative because it was not corrected for submerged whales or for whales that were at the surface but not seen by observers. Most sightings of whales were made in Repulse Bay and Frozen Strait.

Dredging of blue mussels ( Mytilus edulis L.) in a Danish sound: stock sizes and fishery-effects on mussel population dynamic

In April 1993, 1994 and 1995 the abundance of blue mussels, Mytilus edulis L., was estimated in Limfjorden, Denmark. The stocks were assessed by using a down-scaled model of a commercial mussel dredge which efficiency was analysed by comparing its samples with others collected by diver. The mean dredge efficiency was 17%. The fishing area in Limfjorden (700 km 2) is divided into 22 fishery zones and mussel stock size was calculated for each zone. From April 1993 to April 1994 the total stock size declined from 771 000 to 616 000 t. In the same period, the exploitation rate in the fishery was 14% of the 1993 stock, and the size of mussel landings from each zone significantly correlated with their change in stock. In April 1995, the total mussel stock was reduced to 494 000 t. The mean exploitation rate in 1994–1995 was 15%. No correlation was observed between the size of mussel landings and the change in the mussel stock. In summer 1994, there was a long period of oxygen depletion in parts of Limfjorden. This caused mortality of 33% of the mussels in the affected areas. In fishery zones without oxygen depletion a 46% increase in the mussel stocks was estimated. The massive loss of blue mussels caused by oxygen depletion exceeds the annual landings of mussels from the fishery.

Krill ( Euphausia superba ) density, proportional and absolute recruitment and biomass in the Elephant Island region (Antarctic Peninsula) during the period 1977 to 1997

The Elephant Island region (Antarctic Peninsula) was selected as a long-term monitoring site to describe the interannual variability of important krill stock parameters. The analysis reviewed and updated krill density and proportional recruitment indices. Krill absolute recruitment and biomass from net sampling surveys are introduced as additional indices from this time series. New survey results from the past two seasons indicate a very successful 1994/1995 year-class and slightly below average proportional recruitment of the 1995/1996 krill year-class. Absolute recruitment of the 1995/1996 year-class was high compared to preceding years, because total stock size was relatively high in 1996/1997. After a period of low krill density and biomass in the area for almost a decade, krill density and biomass have increased. Caution is expressed as to whether this observed increase represents a long-term recovery of the stock.