Survey Catch Rates Research Articles

Coupling state‐of‐the‐art modelling tools for better informed Red List assessments of marine fishes

Abstract In the face of biodiversity loss worldwide, it is paramount to quantify species' extinction risk to guide conservation efforts. The International Union for the Conservation of Nature (IUCN)'s Red List is considered the global standard for evaluating extinction risks. IUCN criteria also inform national extinction risk assessments. Bayesian models, including the state‐of‐the‐art JARA (‘Just Another Red List Assessment’) tool, deliver probabilistic statements about species falling into extinction risk categories, thereby enabling characterisation and communication of uncertainty in extinction risk assessments. We coupled the state‐of‐the‐art VAST (‘Vector Autoregressive Spatio‐Temporal’) modelling tool and JARA, for better informed Red List assessments of marine fishes. In this framework, VAST is fitted to scientific survey catch rate data to provide indices to JARA whose uncertainty is propagated to JARA outcomes suggesting extinction risk categories (under the population reduction criterion). In addition, VAST delivers a valuable habitat assessment to better understand what may be driving extinction risk in the study region. Here, we demonstrate the coupled VAST‐JARA modelling framework by applying it to five contrasting North Sea species, with or without a quantitative stock assessment and with different conservation statuses according to the latest global Red List assessments. The North Sea application coupled with previous assessments and studies suggest that, among the three elasmobranchs, starry ray is in most need of urgent research (and conservation actions where appropriate), followed by spurdog, while lesser‐spotted dogfish is increasing in biomass. Moreover, both the VAST‐JARA modelling framework and previous research indicate that, while European plaice is not of conservation concern, cod has likely met the IUCN criteria for being listed as Endangered recently. Synthesis and applications. The predictions of the VAST‐JARA modelling framework for North Sea species, including JARA output and VAST habitat assessment, constitute valuable supporting information to make interpretations based on Red List guidelines, which will help decision‐makers in their next North Sea Red List assessment. We foresee applications of the modelling framework to assist Red List assessments of numerous marine fishes worldwide. Our modelling framework has many potential advantageous uses, including informing resource management about climate change impacts on species' extinction risks.

An integrated catch-at-age model for analyzing the variability in biomass of Pacific sardine (Sardinops sagax) from the Gulf of California, Mexico

Knowledge of the population dynamics and changes in the abundance of Pacific sardine fishery is relevant for fishery management. In this study, an integrated catch-at-age model (ASAP) was performed on 47 fishing seasons (1971−2018) ofSardinops sagaxin the Gulf of California, including five indices of relative abundance (survey catch rate, acoustic surveys, egg and larvae, spawning probability, and bird diet index). The ASAP model showed the importance of age group 0 (recruits); this is the most important age group in the Pacific sardine fishery, and its abundance has consequences for the variability of the total abundance of the species. In this study, the spawning and acoustic indices denoted the changes in abundance of Pacific sardine population better than the survey catch rate and bird diet ratio. Historically, the total biomass is estimated to have varied from 461 thousand to 4.82 million t. In the 1970s and 1980s, a peak biomass close to three million tons was reached, and in the second peak from the mid-1990s to the late 2000s, the total biomass varied between 3.26 and 4.82 million t. When the spawning biomass was at least 750 x 103t, the recruitment (age group 0) showed high values, promoting high catches in the subsequent years. The annual fishing mortality estimates did not show a rise related to the periods of high harvestable biomass; therefore, the catch records were low in comparison to the harvestable biomass estimated from ASAP, and the annual fishing mortalities were less than 0.22 during periods of high abundance. The changes in biomass and the variability observed in the Pacific sardine fishery were validated for the model diagnostics; the retrospective patterns did not identify poor fits to data, therefore the robustness of the model can be used for management purposes. According to the Kobe phase plot, the Pacific sardine fishery in the Gulf of California has been fully exploited with an apparent stable level of harvest.

Nonlinearity and Spatial Autocorrelation in Species Distribution Modeling: An Example Based on Weakfish (Cynoscion regalis) in the Mid-Atlantic Bight

Nonlinearity and spatial autocorrelation are common features observed in marine fish datasets but are often ignored or not considered simultaneously in modeling. Both features are often present within ecological data obtained across extensive spatial and temporal domains. A case study and a simulation were conducted to evaluate the necessity of considering both characteristics in marine species distribution modeling. We examined seven years of weakfish (Cynoscion regalis) survey catch rates along the Atlantic coast, and five types of statistical models were formulated using a delta model approach because of the high percentage of zero catches in the dataset. The delta spatial generalized additive model (GAM) confirmed the presence of nonlinear relationships with explanatory variables, and results from 3-fold cross-validation indicated that the delta spatial GAM yielded the smallest training and testing errors. Spatial maps of residuals also showed that the delta spatial GAM decreased the spatial autocorrelation in the data. The simulation study found that the spatial GAM over competes other models based on the mean squared error in all scenarios. That indicates that the recommended model not just works well for the NEAMAP survey but also for other cases as in the simulated scenarios.

Quantitative Analysis of the Fishery-Independent Queensland Saucer Scallop (Ylistrum balloti) Trawl Survey

The Queensland saucer scallop Ylistrum balloti fishery is a significant component of the Queensland East Coast Otter Trawl Fishery. In 1996, the saucer scallop stock collapsed, and by 2006, had partially recovered. In 2016, the stock again collapsed, when it was declared, and remains, overfished. In response to the collapses, fishery managers implemented an annual fishery-independent trawl survey of scallop abundance, although funding for the survey has been inconsistent. This study examined 8 y of discontinuous fishery-independent scallop trawl survey data (1997 to 2000, 2017 to 2020). A two-stage approach was introduced to firstly calibrate catch rates for differences between survey vessels for each year, and then derive adjusted mean densities (number ha–1) for the 0+ and 1+ scallop age classes and total scallops, for each year and strata. Quasi-Poisson generalized linear models were used for both stages. Results showed that adjusted mean total scallop densities declined by approximately half from the early survey years (1997 to 2000) to the later survey years (2017 to 2020). Adjusted mean total scallop density and adjusted mean densities of the 0+ and 1+ age classes were lowest in 2020. The 1+ age class numerically dominated survey catch rates.

Fine-scale environmental effects on Cape hake survey catch rates in the northern Benguela, using data from a trawl-mounted instrument package

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 584:185-198 (2017) - DOI: https://doi.org/10.3354/meps12350 Fine-scale environmental effects on Cape hake survey catch rates in the northern Benguela, using data from a trawl-mounted instrument package Paulus Kainge1,2,*, Kai Wieland2 1National Marine Information and Research Centre (NatMIRC), Ministry of Fisheries and Marine Resources, Swakopmund, Namibia 2DTU Aqua - National Institute of Aquatic Resources, Technical University of Denmark, 9850 Hirtshals, Denmark *Corresponding author: paulus.kainge@mfmr.gov.na ABSTRACT: We investigated fine-scale effects of environmental variables associated with habitat distribution for 4 size groups of Cape hakes, Merluccius capensis and M. paradoxus, using generalized additive models (GAMs) with a negative binominal error distribution. This study took place during the Namibian hake trawl survey of 2016, and was made possible for the first time in Namibia by collecting oceanographic information with a trawl-mounted instrument package concurrently with the catch data. Depth, geographical position, bottom oxygen and bottom temperature had the most pronounced effect on the catch rates of both hake species, whereas solar zenith angle representing diel effects and surface layer chlorophyll appeared to be less important. The explained deviance for the best models ranged from 71.4% for M. capensis to 92.7% for M. paradoxus between 43 and 57 cm in length. Differences in catch rates between species and size groups were most pronounced for bottom depth and bottom oxygen. The results show the potential value of trawl-mounted instrumental packages for the collection of reliable environmental data important in the study of environmental influence on abundance, catch rates and distribution, and in turn in the assessment and management of a resource. KEY WORDS: Survey catchability · Bottom trawl catch rates · Benguela current system · Green water effect · Negative binomial GAM · Full text in pdf format Supplementary material PreviousNextCite this article as: Kainge P, Wieland K (2017) Fine-scale environmental effects on Cape hake survey catch rates in the northern Benguela, using data from a trawl-mounted instrument package. Mar Ecol Prog Ser 584:185-198. https://doi.org/10.3354/meps12350 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 584. Online publication date: December 07, 2017 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2017 Inter-Research.

Diel Variations in Survey Catch Rates and Survey Catchability of Spiny Dogfish and their Pelagic Prey in the Northeast U.S. Continental Shelf Large Marine Ecosystem

This study examines the potential uncertainty in survey biomass estimates of Spiny Dogfish Squalus acanthias in the Northeast U.S. Continental Shelf Large Marine Ecosystem (NES LME). Diel catch-per-unit-effort (CPUE) estimates are examined from the Northeast Fisheries Science Center bottom trawl surveys conducted during autumn (1963–2009) and spring (1968–2009). Influential environmental variables on survey catchability are identified for Spiny Dogfish life history stages and five pelagic prey species: Butterfish Peprilus triacanthus, Atlantic Herring Clupea harengus, shortfin squid Illex spp., longfin squid Doryteuthis spp., and Atlantic Mackerel Scomber scombrus. Daytime survey catchability was significantly higher than nighttime catchability for most species during autumn and for mature male Spiny Dogfish, shortfin squid, and longfin squid during spring in the NES LME. For most stages and species examined, breakpoint analyses identified significant increases in CPUE in the morning, peak CPUE during the day, and significant declines in CPUE in the late afternoon. Seasonal probabilities of daytime catch were largely driven by solar zenith angle for most species, with stronger trends identified during autumn. Unadjusted CPUE estimates appear to overestimate absolute abundance, with adjustments resulting in reductions in absolute abundance ranging from 41% for Spiny Dogfish to 91% for shortfin and longfin squids. These findings have important implications for Spiny Dogfish regarding estimates of population consumption of key pelagic prey species and their ecological footprint within the NES LME. Received February 25, 2015; accepted December 16, 2015

Contribution of Hatchery‐Reared Walleyes to Populations in Northern Green Bay, Lake Michigan

AbstractThe effectiveness of stocking hatchery‐reared Walleyes Sander vitreus to supplement native populations in large, open systems like the Great Lakes has not been thoroughly evaluated. I quantified recent contributions of stocked Walleye fingerlings to populations in Little Bay de Noc (LBDN) and Big Bay de Noc (BBDN) in northern Green Bay, Lake Michigan. Oxytetracycline‐marked Walleye fingerlings were stocked in June, and late summer gill‐net and night‐time boat electrofishing surveys were used to index Walleye year‐class abundance and collect juvenile Walleyes for hatchery mark evaluation. For the 2004–2009 year‐classes, 76% of the age‐0 to age‐3 Walleyes examined from LBDN were of wild origin and 62% in BBDN were naturally reproduced fish. Survey catch rates of juvenile Walleyes were similar for stocked and nonstocked year‐classes. Assessment catch rates of age‐1 and age‐2 Walleyes differed significantly by location, with average catch rates in LBDN often being ten times higher than those in BBDN. Age‐0 Walleyes persisted to older ages and were well‐represented at numerous sampling locations in LBDN, but few age‐1 and older Walleyes were caught in BBDN. The differences in growth between hatchery‐reared and wild Walleyes were minor compared with the differences between bays. Based on stocking records and creel estimates available since 1985, the harvest rate of Walleyes was not significantly correlated to the numbers of Walleyes stocked 4–6 years earlier in LBDN or BBDN. Despite low stocking rates, stocked fish likely provided some contribution (though not a statistically significant one) to Walleye year‐classes and the sport fishery in LBDN, but their contribution in BBDN was less apparent. Managers should weigh the trade‐offs of supplemental stocking in Great Lakes waters when considering requests for hatchery Walleyes in smaller lakes and rivers, especially when stocking resources are limited.

Improving fishery-independent indices of abundance for a migratory walleye population

The primary goal of many biological surveys is to provide an unbiased representation of trends of population abundance. However, there are often factors other than abundance that vary over time and influence catch rates and thus inferences about population trends from surveys. This is particularly true for highly mobile species because of interannual variation in the timing, extent, and duration of movements, and for surveys that are not randomized with respect to space and time. We developed general and generalized linear mixed models to standardize Canadian and United States fishery-independent surveys that provide an index of basin-level walleye (Sander vitreus) population abundance trends in Lake Erie (1983–2008). In Canadian waters, the probability of a non-zero catch was associated with the type of gill net set (canned>bottom sets), the presence of hypoxia (negative trend; −), and secchi depth (−). Positive catch rates were associated with the set type (canned>bottom) and water depth (+). In United States waters, survey catch rates were associated with secchi depth (−) and surface water temperature (+). For each case, the best model included random effects (interactions between year, week, basin, sub-basin) which accounted for a modest amount of the total variation. General abundance trends were similar between the standardized and nominal indices, but substantial annual variation in the direction and magnitude of the difference between indices was observed. We recommend the use of standardized indices for walleye population assessments because these account for factors influencing catch rates other than changes in abundance.

Feeding Habits of European Hake, <I>Merluccius Merluccius</I> (Actinopterygii: Gadiformes: Merlucciidae), from the Northeastern Mediterranean Sea

European hake, Merluccius merluccius (Linnaeus, 1758), is an important predator of deep Mediterranean upper shelf slope communities, being a nektobenthic species inhabiting a wide depth range (20–1000 m) throughout the Mediterranean Sea and the northeastern Atlantic region (Carpentieri et al. 2005). It is one of the chief commercial and most heavily exploited species of demersal fishery in all northern Mediterranean countries. Recent time-series studies referring to the western part of the Adriatic Sea have shown catches to be made up mainly of specimens shorter than 20 cm TL, with survey catch rates apparently increasing between 1985 and 1995 and decreasing in the following years both in the northern(Piccinetti and Piccinetti Manfrin 1971, Manfrin et al. 1998) and southern Adriatic Sea (Marano et al. 1998). In 2006, annual hake landings were estimated to be around 76 000 t in the Mediterranean (Anonymous 2008) and around 18 000 t in the Adriatic Sea (Anonymous 2007), with the species being the most abundant in the demersal group of the Adriatic Sea (Ungaro et al. 2001). As a rule, hake feeds predominantly on fish and crustaceans, and the proportion of piscivory increases with hake length; crustaceans appearing mostly in the stomach of <16 cm hakes in the northern-central Adriatic Sea (Karlovac 1959, Županovic 1968, Piccinetti and Piccinetti Manfrin 1971, Jukic 1972, Froglia 1973, Jardas 1976). The presently reported study analysed the diet of the hake in the northeast Mediterranean, which, given its abundance, plays an important role in comprehending the food chain dynamics. Despite hake’s environmental and economic importance (Oliver and Massuti 1995) in the Mediterranean, much of its ACTA ICHTHYOLOGICA ET PISCATORIA (2011) 41 (4): 277–284 DOI: 10.3750/AIP2011.41.4.03

Multiscale influence of climate on estuarine populations of forage fish: the role of coastal upwelling, freshwater flow and temperature

ABSTRACT: We examined how local- and regional-scale environmental drivers affect patterns ofabundance and recruitment in 2 abundant and ecologically significant forage fishes (Pacific herring Clupea pallasi and surf smelt Hypomesus pretiosus) in the Skagit River estuary (Puget Sound, Wash-ington, USA). We identified associations between survey catch rates and environmental conditions at2 scales: within-season distributional shifts in response to local environmental conditions, and inter-annual patterns of relative year class strength related to both local- and regional-scale drivers. Usingmonthly data that spanned a 9 yr period, we found that a small proportion (<2%) of the total deviancein catch rates for both species was related to within-estuary variation in surface water temperatureand salinity but that a larger fraction (7 and 12% for Pacific herring and surf smelt, respectively) wasexplained by interannual variation in recruitment strength. Annual abundance indices for both spe-cies were uncorrelated with cumulative river discharge and regional sea surface temperature butpositively correlated with an index of cumulative coastal upwelling, suggesting a linkage betweenregional-scale environmental conditions and age-0 recruitment. Moreover, our annual age-0 Pacificherring time series was positively correlated with a similar time series from the Strait of Georgia(~100 km north), further suggesting that age-0 recruitment in these populations is synchronized byregional upwelling as opposed to estuary-specific environmental forcing related to river flows. Thepresent study isolates a potential key process governing age-0 forage fish abundance in this systemand highlights the importance of simultaneously evaluating patterns of variability across multiplespatiotemporal scales in order to identify the primary pathways through which climate may impactestuarine populations.KEY WORDS: Puget Sound · Pacific herring · Surf smelt · Deviance partitioning · Upwelling ·Fish recruitment

Feeding habits and niche separation among the deep-sea chimaeroid fishes Harriotta raleighana, Hydrolagus bemisi and Hydrolagus novaezealandiae

The diets of Harriotta raleighana, Hydrolagus bemisi and Hydrolagus novaezealandiae were determined from examination of digestive tract contents from specimens sampled at depths of 219 to 876 m on the submarine ridge, Chatham Rise, New Zealand. All 3 species fed predominantly on benthic epifauna and infauna. Similarity percentages indicated the diet of H. raleighana was characterised by polychaetes and molluscs, H. bemisi by crabs and polychaetes, and H. novaezealandiae by crabs and sea urchins. Multivariate analyses using distance-based linear models found the most important predictors of diet variability were a categorical predictor for the subtropical front (STF) and bottom temperature. Cluster analysis of species subgroups indicated there was a greater similarity in diet between H. raleighana and H. bemisi, and between H. bemisi and H. novaezealandiae, in similar locations, than between subgroups of H. bemisi in different locations. Generalised additive models indicated that depth was the most important spatial predictor of research trawl survey catch rates, and depth separation was most pronounced between H. raleighana and H. novaezealandiae. Other significant predictors of diet variability included longitude, and fish length or weight. In all 3 species the diet of smaller fish included more polychaetes, small crustaceans and gastropods, and the diet of larger fish contained more decapod crustaceans. The primary factor determining niche separation among the 3 species appears to be depth and distribution rather than dietary specialisation.

Throwing in the towel: When do adverse conditions dictate a weather day during a bottom trawl survey?

Abstract Fishermen and field biologists routinely work under difficult conditions. It is natural in this setting to continue fishing during bad weather as long as safety is not compromised. However, as conditions deteriorate, the effectiveness of trawl survey gear may be reduced well before safety considerations force the cessation of sampling activity. Through comparison of 1754 hauls conducted on chartered vessels between 2003 and 2005 on the shelf and slope of the U.S. West Coast, we show a significant reduction in catch rates of bottom dwelling fish and invertebrates as a function of visually estimated wave height. Comparing randomly located trawl sets that occurred fortuitously close to one another, we conclude that to reduce the estimated effect of wave height on benthic species catch rates (and potentially relative biomass estimates) survey trawl activities on these vessels should be halted when wave heights reach six feet (1.83 m). Only 41 hauls from the 2003–2005 data were conducted when wave heights exceeded five feet (1.52 m), however their exclusion reduces the maximum expected bias in any single tow due to wave height by ∼31%. Further reductions in maximum wave height for routine sampling would begin to exclude geographic areas of the survey introducing a new bias to the randomized design. We further encourage the routine electronic measurement of vertical motion due to wave action during sampling so that future analyses can improve our understanding of its effect on trawl survey catch rates.

Sperm whale depredation of sablefish longline gear in the northeast Pacific Ocean

AbstractInteractions between marine mammals and fisheries include competition for prey (catch), marine mammal entanglement in fishing gear, and catch removal off fishing gear (depredation). We estimated the magnitude of sperm whale depredation on a major North Pacific longline fishery (sablefish) using data collected during annual longline surveys. Sperm whale depredation occurs while the longline gear is off‐bottom during retrieval. Sperm whales were observed on 16% of longline survey sampling days, mostly (95% of sightings) over the continental slope. Sightings were most common in the central and eastern Gulf of Alaska (98% of sightings), occasional in the western Gulf of Alaska and Aleutian Islands, and absent in the Bering Sea. Longline survey catches were commonly preyed upon when sperm whales were present (65% of sightings), as evidenced by damaged fish. Neither sperm whale presence (P= 0.71) nor depredation rate (P= 0.78) increased significantly from 1998 to 2004. Longline survey catch rates were about 2% less at locations where depredation was observed, but the effect was not significant (P= 0.34). Estimated sperm whale depredation was &lt;1% of the annual sablefish longline fishery catch off Alaska during 1998 to 2004.

Research assessment for the management of the industrial shallow-water multi-species shrimp fishery in Sofala Bank in Mozambique

The industrial shallow-water shrimp fishery at Sofala Bank whose annual average catch of 8600 t is valued at about US$ 80 million, is one of the main industries in Mozambique. The shrimps are also fished by an artisanal and semi-industrial fishery. The industrial fleet is managed by a combination of limiting the number of vessels, catch quota and seasonal closures. The main species caught are Penaeus indicus and Metapenaeus monoceros with average catches of 4600 and 2500 t, respectively. The other species ( P. japonicus, P. latisulcatus and P. monodon) have increased from 10% of the catch in the 1980s to 20% since 1992 due to increased night fishing. Standardized catch rates for all species showed a high level of variation between vessels, 200–650 kg/day, that are related to vessel length ( r = 0.78). The fishing power increased linearly for vessels from 14 to 30 m with little increase from 30 to 40 m. The mean weight of P. indicus increased from March to August and then declined as new recruits enter the fishery. While the mean weight of capture by December was similar to that at the start of the fishery in March, the relative numbers caught were about 20 times greater in March compared to December. The spatial distribution of the mean shrimp weight at the time of recruitment in March shows that smaller shrimps occur inshore in shallow water 5–15 m. The annual variation in the P. indicus and M. monoceros catch can be explained by a pre-season recruitment survey catch rate and fishing effort ( r = 0.81, n = 8, p = 0.07) and monthly run-off of the Zambeze River in the previous wet season ( r = 0.64, n = 19, p < 0.01), respectively. This study provides a basis for improved stock assessment and management by: (a) standardization of fishing effort; (b) greater effective effort control through control of increases in fishing power by vessel size or controls on trawl net sizes; (c) extension of seasonal closures to include March based on the recruitment pattern of the main species; (d) use of the prediction of catches in the management of the fishery; (e) temporary (March–April) or permanent spatial closures of the inshore shallow waters based on the introduction of the vessel monitoring system to protect juvenile shrimps.

Stock evaluation of giant scallop (Placopecten magellanicus) using high-resolution acoustics for seabed mapping

Abstract Survey designs in use for the evaluation of sea scallop stocks do not consider the variability of sediment type, despite strong evidence of its importance for the recruitment and survival of scallops on the sea floor. This study examines the distribution of scallops on Browns Bank, Scotian Shelf, at two test sites, in comparison to sea floor sediment distribution, with particular attention to the effects of small-scale sediment variability on the abundance of the commercially exploited scallop. Important links between scallop abundance, sediment type and habitat structure are described. Scallops are strongly associated with gravel lag deposits, which are readily distinguishable from sand-covered terrain through the use of multibeam backscatter data. There exists a highly significant correlation between scallop survey catch rates and backscatter intensity which can be used for the prediction of scallop stock abundance. Developments in underwater acoustics enable for more precise sea floor mapping and contribute to better estimates of scallop abundance.

Timing of Atlantic cod (Gadus morhua L.) seasonal migrations in the southern Gulf of St Lawrence: interannual variability and proximate control

Comeau, L. A., Campana, S. E., and Chouinard, G. A. 2002. Timing of Atlantic cod (Gadus morhua L.) seasonal migrations in the southern Gulf of St Lawrence: interannual variability and proximate control. – ICES Journal of Marine Science, 59: 333–351. The objective of this study was to identify likely proximate cues to the seasonal migrations of cod, Gadus morhua L., in the southern Gulf of St Lawrence. We computed a migration timing index across several years using commercial fishery (1970–1992) and sentinel survey (1995–1999) catch rate data, and closely monitored the seasonal changes in temperature, dissolved oxygen, food intake, and energy stores between 1995 and 1998. The inter-year variability in the timing of the seasonal migrations was relatively low (s.d.=8 d for spring migration, 10 d for autumn migration), suggesting that photoperiod was involved in the control mechanism of migration. However, other factors were also implicated in the control mechanism, given that cod initiated the autumn migration progressively earlier in the late 1980s and 1990s. At the onset of the autumn migration, dissolved oxygen concentrations were above levels known to induce avoidance behaviour in this species, and food resources were considered to be relatively abundant. Similarly, cod did not initiate the autumn migration at a specific temperature or following a particular temperature regime experienced during summer. However, cross correlation function analysis indicated that a widespread cooling of near bottom waters preceded the mid-1980s shift in migration timing by one to two years, suggesting that the cooling event was linked in some way to the change in migration dates. Other correlations showed that the earlier migrations coincided with older cod becoming more abundant and smaller in size, and also with their principal fish prey (herring and capelin) increasing in number. These results suggest that older cod played a key role in launching the early autumn migrations. One plausible interpretation is that their decline in body size and lipid-rich diets led to a rapid build up of energy reserves over summer grounds, and thus to an earlier readiness to engage in the autumn migration. We discuss the potential impact of earlier departures on stock productivity. 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd.

Spatial patterns of whiting abundance in Scottish waters and relationships with environmental variables

This paper uses objective classification methods, a combination of principal components analysis (PCA) and cluster analysis, applied to long-term average data, to define areas of similar seasonal patterns of whiting abundance in Scottish waters, based on fishery data on landings and effort (by month and by ICES rectangle). A geographic information system (GIS) is used to qualitatively describe the relationships of these spatial patterns of whiting abundance with (a) trawl survey catch rates by age-class, and (b) environmental factors. The results show that the spatial patterns of whiting abundance are related to age, as well as to depth and to spatial patterns of sea surface temperature (SST) in winter. The area (within the North Sea) of highest whiting abundance, and the largest seasonal change in whiting abundance, corresponds to the area of highest survey catch rates for older (>2 years old) whiting in winter, a particular spatial pattern of SST in winter and water depths of 100–200 m. This spatial pattern of SST may indicate an effect of the inflow of the North Atlantic water. This study provides a guide for selecting study areas for future quantitative analysis and the methods proposed may also provide a useful management tool.

Effect of water temperature on catchability of Atlantic cod (Gadus morhua) to the bottom-trawl survey in the southern Gulf of St Lawrence

Correlations between catch rates in bottom-trawl surveys and indices of environmental conditions or fish distribution have been attributed to effects of the environmental conditions or fish distribution on catchability to the survey. We tested this hypothesis using data on cod in the southern Gulf of St Lawrence. Survey catch rates of cod were significantly correlated with indices of bottom temperature and of cod temperature and depth distributions. Correlations were in the directions expected on the basis of predicted effects on catchability or availability to the survey. However, tests involving calibrations of sequential population analysis (SPA) or residuals from multiplicative analyses of the survey catch rates (with terms for yearclass, age and cumulative mortality) did not support the hypothesis that these correlations resulted from effects on catchability to the survey. These tests provided no support for an effect of cod temperature or depth distribution on catchability. They provided some support for an effect of bottom temperature conditions on availability to the survey, but this effect on availability was not reflected by the correlations observed between bottom temperature indices and survey catch rates. We conclude that adjustments for effects on catchability should be based on relationships with inconsistencies in survey catch rates instead of relationships with the catch rates themselves. These adjustments could be incorporated in calibration of the SPA or calculated based on relationships with residuals from models relating survey catch rate to yearclass, age and fishing mortality. However, such adjustments were negligible in the case of southern Gulf of St Lawrence cod. 2000 International Council for the Exploration of the Sea

Comparing 1994 Angler Catch and Harvest Rates from On-Site and Mail Surveys on Selected Maine Lakes

Fishery managers use data on catch and harvest rates collected with both on-site and off-site angler surveys. Many researchers have hypothesized that catch rates from these surveys will differ systematically due to various biases. However, few direct comparisons have been made between the two survey methods using the same waters and seasons. We compared catch and harvest rates for three coldwater species, individually and combined, from concurrent mail and on-site surveys on five Maine lakes during the 1994 ice fishing season. Comparisons were also made for two Maine lakes during the 1994 open-water season. Estimates of catch and harvest from the mail survey exceeded the rates from on-site surveys in 28 of 38 comparisons for individual species. The average on-site to mail survey ratio was 0.40 for catch rates and 0.66 for harvest rates but varied significantly by lake, season, and species. The results suggest that mail surveys do not produce accurate estimate of catch and harvest rates. In situations where only mail survey data are available, fisheries managers should recognize that mail survey catch and harvest rates are likely to be overestimates compared with on-site data.

Sustainability of flatfish-dominated fish assemblages in Hecate Strait, British Columbia, Canada

Species catch-rate data from a series of four trawl surveys of the Hecate Strait region in British Columbia, on the Pacific continental shelf of Canada were analysed using agglomerative clustering techniques. Four relatively stable fish assemblages were identified. These assemblages were dominated by flatfish species and characterized by depth and bottom type. Although the relative abundances of component species within assemblages did change seasonally, we observed little change in terms of which species were dominant. Geographical boundaries of the assemblages were also relatively stable over the four surveys completed to date. Persistent boundaries were found in the 50–60 m range and 130–140 m range. We identified both regular and seasonal components of assemblages. Arrowtooth flounder and Pacific sanddab showed remarkable stability in their overall distributions, leading us to suspect that survey catch rates for these species would be suitable for monitoring changes in abundance. We found no significant differences in either component abundance or diversity of assemblages under two widely different fishing effort regimes in different portions of the Strait.