Impact on biodiversity of scallop dredging in San Matías Gulf,northern Patagonia (Argentina)

Between 1968 and 1994, Tehuelche scallop (Aequipecten tehuelchus) was fished in San Matías Gulf (Argentine Patagonia) using dredges. The catch was not sorted on board; epifaunal invertebrate bycatch and inorganic substrate removed by the dredges were landed together with scallops. Surveys were conducted in 1987 and 1997 on four fishing grounds following the same methodology to estimate the abundance of epifaunal components using catch and swept-area data. Univariate, distributional, and multivariate methods were used to analyze biodiversity and its change between surveys. There was no recorded dredging on two of the fishing grounds in the intervening period between the two surveys. Fishing effort in the order of 5,000 effective fishing hours occurred at each of the other two grounds, during that period. Multivariate analysis indicates that macrofaunal assemblages changed in each ground between 1987 and 1997, according to fishing intensity and time lapsed since last fishing action. Fishing grounds that were not dredged during the 10-year period showed no significant changes in species composition and dominance. The results support the hypothesis that dredging causes high levels of disturbance that affect the whole benthic ecosystem. Community “recovery” seems to be a non-stationary process that may not reverse the system to pre-harvest conditions, but lead instead to a sequence of new states with faunistic changes governed by colonization and slow rebuilding of habitat complexity. Handling editor: K. Martens     

“First” abyssal record of Stenosemus exaratus (G.O.?Sars, 1878) (Mollusca,Polyplacophora) in?the?North-Atlantic Ocean

The first proven abyssal record of Stenosemus exaratus (G.O. Sars, 1878) is presented on the basis of an ROV study in the Irish Sea. For the first time in situ images of the species and data on the environmental parameters are provided.     

The broad-scale distribution of five jellyfish species across a temperate coastal environment

Jellyfish (medusae) are sometimes the most noticeable and abundant members of coastal planktonic communities, yet ironically, this high conspicuousness is not reflected in our overall understanding of their spatial distributions across large expanses of water. Here, we set out to elucidate the spatial (and temporal) patterns for five jellyfish species (Phylum Cnidaria, Orders Rhizostomeae and Semaeostomeae) across the Irish & Celtic Seas, an extensive shelf-sea area at Europe’s northwesterly margin encompassing several thousand square kilometers. Data were gathered using two independent methods: (1) surface-counts of jellyfish from ships of opportunity, and (2) regular shoreline surveys for stranding events over three consecutive years. Jellyfish species displayed distinct species-specific distributions, with an apparent segregation of some species. Furthermore, a different species composition was noticeable between the northern and southern parts of the study area. Most importantly, our data suggests that jellyfish distributions broadly reflect the major hydrographic regimes (and associated physical discontinuities) of the study area, with mixed water masses possibly acting as a trophic barrier or non-favourable environment for the successful growth and reproduction of jellyfish species. Handling Editor: K. Martens     

The influence of spawning and nursery grounds on the distribution of sole Solea solea (L.) in the Irish Sea, Bristol Channel and adjacent areas

In the coastal waters of west England and Wales, sole, Solea solea (L.), are found in greatest abundance in the north-east Irish Sea and the Bristol Channel. Data from research and charter vessel surveys in these areas showed differences in the spatial distribution of age-groups between the two regions. Juvenile sole (0-group, 1-group and 2-group) were found almost exclusively in the shallow (<20 m deep) parts of the north-east Irish Sea and adults (3-group and older) were also found in the same area and in deeper water. In contrast, while juvenile sole in the Bristol Channel were abundant in and around Carmarthen Bay, Swansea Bay and the Severn estuary, they also occurred frequently in deeper areas of the Bristol Channel down to depths of 40 m. Adult sole in the Bristol Channel were also relatively more abundant at depths >40 m than adults in the Irish Sea. The movements of sole which produce these patterns of distribution in the two areas are also shown by the recaptures of sole tagged in the respective nursery areas. The differences in the distribution of sole in the two areas may be related to the depths of their spawning grounds and the distances that maturing fish must travel to reach them.

In the north-east Irish Sea, sole spawning grounds are generally in waters of <40 m depth and within an extensive area of relatively shallow and gently shelving sediments close to the coastal nursery grounds. The bottom topography of the Bristol Channel is more steeply shelving and the spawning grounds of sole are at a depth of 40–75 m off Trevose Head, ≈100 km from the main nursery areas. The hydrographic conditions in the two areas are described, and it is concluded that one of the more important requirements of a successful spawning ground is the presence of suitable hydrographic conditions to transport eggs and larvae to nursery areas.     

Effects of sponge and barnacle encrustation on survival of the scallop <Emphasis Type="Italic">Chlamys hastata</Emphasis>

The scallop Chlamys hastata frequently carries epibionts such as sponges and barnacles on its shells. Although the scallop-sponge relationship has been characterized as a mutualism, little is known about the scallop-barnacle relationship. This study investigated the effects of sponge and barnacle encrustation on the ability of C. hastata to avoid predation by the sea star Pycnopodia helianthoides. In feeding trials, P. helianthoides caught and consumed significantly more barnacle-encrusted scallops (7.7 ± 0.8 out of 20 scallops) than scallops encrusted by either of the sponges Myxilla incrustans (4.1 ± 0.9) or Mycale adhaerens (3.0 ± 0.5). Epibiont-free scallops (5.7 ± 0.5) formed an intermediate treatment between barnacle-encrusted and sponge-encrusted scallops. Possible mechanisms by which the sponges protected the scallops were investigated in two ways: two feeding trials were videotaped to allow qualitative analysis of sea star and scallop behavior and sea star feeding responses to scallop and sponge homogenates were determined to investigate if sea stars accept scallops and sponges as prey. Sea stars displayed positive feeding responses to scallop puree 97.5% ± 1.6 of the time while only displaying positive responses to Mycale adhaerens homogenate 4.4% ± 2.0 of the time and to Myxilla incrustans homogenate 4.4% ± 2.9 of the time. The videotaped feeding trials indicated that interference with tube feet adhesion by the sponge deterred predation. Observations of both sea stars that were videotaped showed that neither avoided trying to capture sponge-encrusted scallops, and at no time was a captured scallop willingly released by the sea stars. Thus, it appears that sponges provide tactile-mechanical protection and possibly chemical or tactile camouflage in this predator/prey relationship. Finally, the effects of sponge encrustation on barnacle settlement were determined. Field experiments showed that barnacle larvae settled more frequently on epibiont-free scallops than on those with either of the two sponges, potentially protecting the scallops from an epibiont that increases the scallop’s susceptibility to predation. Handling editor: K. Martens     

Influence of proximal stimuli on swimming in the sea hare Aplysia brasiliana

Although the neurobiology and physiology of sea hares are extensively studied, comparatively little is known about their behaviour or ecology. Several species of sea hares swim, but the function of swimming is unclear. In this paper, we tested the hypotheses that swimming in Aplysia brasiliana serves to find food and mates, and to escape predators. Our data strongly support the hypothesis that swimming in A. brasiliana is related to feeding. Sea hares deprived of food overnight swam 12 times longer than ones that had been fed. When sea hares contacted food while swimming they invariably stopped, while those contacting a plastic algal mimic mostly continued to swim. Our experiments provided no evidence to support the hypothesis that swimming in sea hares is related to social behaviour. Sea hares deprived of copulatory mates for 3 days did not swim longer than ones held in copulating groups. Moreover, swimming sea hares never stopped swimming upon encountering a conspecific. Our experiments also supported the hypothesis that swimming in sea hares is related to predation. Sea hares stimulated with a standardised tail pinch and exposed to ink of conspecifics swam four times longer than control individuals, and tail-pinched sea hares that released ink swam five times longer than ones that did not release ink. However, because predators of adult sea hares are mostly lacking and because sea hares often swim spontaneously without predators being present, we conclude that swimming behaviour in A. brasiliana is primarily related to food-finding.     

Accuracy of catch-effort methods for estimating fish density and biomass in streams

Synopsis At each of 11 localities a section of stream was closed off with nets and an electrofisher used to estimate the abundance of fishes in the section. Each section was fished from 5–7 times with each fishing equalling one unit of effort. Using the catch-effort methods of Leslie, DeLury and Ricker, separate estimates were made for each species. In several cases species were split into size groups and estimates made for each group. The fish remaining in each section after the fishings were collected using rotenone. Thus the estimates could be compared to the actual number of fish present. Estimates were considered to be either good, if the regressions used in the above methods were statistically significant or bad if they were not significant. Lower limits for the number of fish and mean weight of a fish for good estimates were identified.The Leslie and Ricker estimates, which did not differ significantly, were least in error. They tended to underestimate (–21.6% on the average for the Leslie method). Direct estimates of biomass did not differ significantly from those made using the estimates for numbers and the mean weight of fish caught. The interrelationships among variables such as mean weight, numbers, catchability, density, biomass, number of catches used, proportion of fish taken during the estimate, number of fish in the last catch and their relationships with the error of the estimates were examined using correlation and principal components analysis. Error was most closely related to the proportion of fish collected. The effects of other variables such as mean weight affected error through catchability and subsequently the proportion of fishes caught. It was not possible to predict a significant proportion of the error using variables which could be measured without a complete collection. The effects of locality, electrofisher, and species on error were examined. Each accounted for a significant proportion of the variability in error but primarily by affecting the proportion of fish caught.These results suggest that the most appropriate way of decreasing error would be to increase the total effort and consequently the proportion of fish collected. This would be best done by increasing the number of fishings used in the estimate.Catchability tended to decrease in successive fishings. The observed trends in changing catchability accounted for most of the error. Size-selectivity, which was evident as a change in mean weight in successive catches, was not significantly associated with changing catchability.     

Standardizing fishery-dependent catch and effort data in complex fisheries with technology change

Standardization of commercial catch and effort data is important in fisheries where standardized abundance indices based on fishery-dependent data are a fundamental input to stock assessments. The goal of the standardization is then to minimize bias due to the confounding of apparent abundance patterns with fishing power. There is a high risk of confounding between fishing power and abundance in fisheries where the fleet has altered their fishing technology over the years. Also, the spatial aspects and the fishing history can be so heterogeneous that any standardization really involves an extrapolation, for example to a hypothetical standard vessel. When the standardization involves an extrapolation, then the appropriate modeling strategy is to build a so-called estimation model, rather than a predictive model. Strategies to build such an estimation model from fishery-dependent data include: pay careful attention to subject matter, and collect information about potential confounding effects to include in the model (putting a high value on the acquisition of data on covariates); model variable catchability at a highly disaggregated scale; aim for realistic coefficients when fitting the model and pay relatively less attention to achieving precision or maximizing explained variance; adopt modern statistical methods to combine data from different sources; and if data are deficient, then apply precautionary allowances. These strategies offer some protection against bias due to confounding, in the absence of formal criteria for identifying the best model.     

206/207PB isotopic ratios applied to particulate transport processes-North eastern Irish Sea

Samples of suspended particulate matter (SPM) were collected from the north eastern Irish Sea, under different tidal conditions. The SPM was extracted with 1M HCl and concentrations of Pb and stable isotopic ratios of Pb (^206/207Pb) determined using atomic absorption spectrometry and ICP-MS. Lead concentrations were found to be in the range 49 to 115 μg g⁻¹ for spring tides and 72 to 148 μg g⁻¹ for neap tides. The^206/207Pb ratios, which were between 1.140 and 1.194, were used to establish possible sources of SPM during the tidal cycle. Samples of surface sediments from along the coastline showed evidence of anthropogenic Pb contamination in the vicinity of Liverpool Bay, with low^206/207Pb ratios of about 1.144, which increased northwards away from the estuary. The results are discussed in terms of the tidally-induced transport of SPM in the coastal boundary zone of the Irish Sea.     

Application of the Kolmogorov-Smirnov test to seasonal phenomena may be inappropriate

The Kolmogorov-Smirnov one-sample test is sometimes used to test seasonality in births or other annually cyclic phenomena. However, it is shown that the maximum deviation from expected (D) may differ by a factor of almost two if the cycle is initiated in different months. Thus, different results are possible from the same data. Unless there is a logical initiation point in a cycle, it is probably better to use other nonparametric statistical tests.     

Dynamic economic analysis on invasive species management: Some policy implications of catchability

The problem of controlling invasive species has emerged as a global issue. In response to invasive species threats, governments often propose eradication. This article challenges the eradication view by studying optimal strategies for controlling invasive species in a simple dynamic model. The analysis mainly focuses on deriving policy implications of catchability in a situation where a series of controlling actions incurs operational costs that derive from the fact that catchability depends on the current stock size of invasive species. We analytically demonstrate that the optimal policy changes drastically, depending on the sensitivity of catchability in response to a change in the stock size, as well as on the initial stock. If the sensitivity of catchability is sufficiently high, the constant escapement policy with some interior target level is optimal. In contrast, if the sensitivity of catchability is sufficiently low, there could exist a threshold of the initial stock which differentiates the optimal action between immediate eradication and giving-up without any control. In the intermediate range, immediate eradication, giving-up without any control, or more complex policies may be optimal. Numerical analysis is employed to present economic intuitions and insights in both analytically tractable and intractable cases.