Midwater Trawl Research Articles

Integrating Ecological and Social Networks in Fisheries

FisheriesVolume 37, Issue 7 p. 291-291 Column: President's Hook Integrating Ecological and Social Networks in Fisheries Bill Fisher, Bill Fisher AFS President wlf9@cornell.edu Search for more papers by this author Bill Fisher, Bill Fisher AFS President wlf9@cornell.edu Search for more papers by this author First published: 12 July 2012 https://doi.org/10.1080/03632415.2012.695995Citations: 1Read the full textAboutRelatedInformationPDFPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessClose modalShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article.Citing Literature Volume37, Issue7July 2012Pages 291-291 RelatedInformation RecommendedSocial Networks and Fisheries: The Relationship between a Charter Fishing Network, Social Capital, and Catch DynamicsKatrina B. Mueller, William W. Taylor, Kenneth A. Frank, John M. Robertson, Dennis L. Grinold, North American Journal of Fisheries ManagementPacific Halibut Bycatch in the Washington, Oregon, and California Groundfish and Shrimp Trawl FisheriesEllen K. Pikitch, John R. Wallace, Elizabeth A. Babcock, Daniel L. Erickson, Mark Saelens, Geir Oddsson, North American Journal of Fisheries ManagementCanada's Recreational Fisheries: The Invisible Collapse?John R. Post, Michael Sullivan, Sean Cox, Nigel P. Lester, Carl J. Walters, Eric A. Parkinson, Andrew J. Paul, Leyland Jackson, Brian J. Shuter, FisheriesThe California Sardine‐Anchovy FisheriesG. B. Talbot, Transactions of the American Fisheries SocietyCharacterization of River Herring Bycatch in the Northwest Atlantic Midwater Trawl FisheriesN. David Bethoney, Kevin D. E. Stokesbury, Bradley P. Schondelmeier, William S. Hoffman, Michael P. Armstrong, North American Journal of Fisheries Management

Fishing management scenarios to rebuild exploited resources and ecosystems of the Northern-Central Adriatic (Mediterranean Sea)

We examined various fishing management options to recover exploited marine resources and ecosystems of the Northern-Central Adriatic Sea. Dynamic simulations were based on a spatial ecological model previously calibrated with time series of data. Scenarios regarding spatial management were evaluated with the establishment of two marine protected areas, respectively, in the Pomo pit and the northern region. In addition, three temporal simulations of temporary closures and overall reduction of fishing effort of demersal and pelagic fleets (bottom, mid-water trawls and purse seines) were also considered. Simulations were run for 45years (1975–2020), including the calibration period (1975–2002), and changes in biomass and catch of marine resources were analyzed. Our results confirm that current fishing management in the Adriatic Sea does not have clear beneficial impacts for the recovery of exploited resources, which will remain depleted in 2020 if “business as usual” continues. Simulations of alternative management suggest that both protected areas could be beneficial for fish population recovery predicting an increase in the biomass of commercial fish and predatory organisms. Simulations of temporary closures and overall reduction of fishing effort also show significant benefits for several commercial resources. We argue that both management measures may be effective tools to recover exploited ecosystems of the Northern-Central Adriatic Sea and halt the decline of marine resources.

Ecological Metrics of Biomass Removed by Three Methods of Purse‐Seine Fishing for Tunas in the Eastern Tropical Pacific Ocean

An ecosystem approach to fisheries management is a widely recognized goal, but describing and measuring the effects of a fishery on an ecosystem is difficult. Ecological information on the entire catch (all animals removed, whether retained or discarded) of both species targeted by the fishery and nontarget species (i.e., bycatch) is required. We used data from the well-documented purse-seine fishery for tunas (Thunnus albacares, T. obesus, and Katsuwonus pelamis) in the eastern tropical Pacific Ocean to examine the fishery's ecological effects. Purse-seine fishing in the eastern tropical Pacific is conducted in 3 ways that differ in the amount and composition of target species and bycatch. The choice of method depends on whether the tunas are swimming alone (unassociated sets), associated with dolphins (dolphin sets), or associated with floating objects (floating-object sets). Among the fishing methods, we compared catch on the basis of weight, number of individuals, trophic level, replacement time, and diversity. Floating-object sets removed 2-3 times as much biomass as the other 2 methods, depending on how removal was measured. Results of previous studies suggest the ecological effects of floating-object sets are thousands of times greater than the effects of other methods, but these results were derived from only numbers of discarded animals. Management of the fishery has been driven to a substantial extent by a focus on reducing bycatch, although discards are currently 4.8% of total catch by weight, compared with global averages of 7.5% for tuna longline fishing and 30.0% for midwater trawling. An ecosystem approach to fisheries management requires that ecological effects of fishing on all animals removed by a fishery, not just bycatch or discarded catch, be measured with a variety of metrics.

The Association of Antarctic Krill Euphausia superba with the Under-Ice Habitat

The association of Antarctic krill Euphausia superba with the under-ice habitat was investigated in the Lazarev Sea (Southern Ocean) during austral summer, autumn and winter. Data were obtained using novel Surface and Under Ice Trawls (SUIT), which sampled the 0–2 m surface layer both under sea ice and in open water. Average surface layer densities ranged between 0.8 individuals m−2 in summer and autumn, and 2.7 individuals m−2 in winter. In summer, under-ice densities of Antarctic krill were significantly higher than in open waters. In autumn, the opposite pattern was observed. Under winter sea ice, densities were often low, but repeatedly far exceeded summer and autumn maxima. Statistical models showed that during summer high densities of Antarctic krill in the 0–2 m layer were associated with high ice coverage and shallow mixed layer depths, among other factors. In autumn and winter, density was related to hydrographical parameters. Average under-ice densities from the 0–2 m layer were higher than corresponding values from the 0–200 m layer collected with Rectangular Midwater Trawls (RMT) in summer. In winter, under-ice densities far surpassed maximum 0–200 m densities on several occasions. This indicates that the importance of the ice-water interface layer may be under-estimated by the pelagic nets and sonars commonly used to estimate the population size of Antarctic krill for management purposes, due to their limited ability to sample this habitat. Our results provide evidence for an almost year-round association of Antarctic krill with the under-ice habitat, hundreds of kilometres into the ice-covered area of the Lazarev Sea. Local concentrations of postlarval Antarctic krill under winter sea ice suggest that sea ice biota are important for their winter survival. These findings emphasise the susceptibility of an ecological key species to changing sea ice habitats, suggesting potential ramifications on Antarctic ecosystems induced by climate change.

Aggregative patterns of pre-spawning Atlantic herring on Georges Bank from 1999-2010

Atlantic herring (Clupea harengus ) in the offshore regions of the Gulf of Maine migrate each fall from their feeding grounds to the northern portion of Georges Bank to spawn. The Northeast Fisheries Science Center’s (NEFSC) herring acoustic survey has taken advantage of this behavior by conducting systematic surveys of the pre-spawning fish each year since 1999. Multi-frequency acoustic and midwater trawl data were collected along transects oriented perpendicular to bathymetric contours. Acoustic backscatter was analyzed to describe the aggregative patterns (e.g., size, location in the water column, and spatial and temporal distribution) of Atlantic herring during these surveys and regression trees were used to examine the aggregation characteristics. The positional variables of distance to spawning grounds and vertical location in the water column were the primary characteristics for describing pre-spawning aggregations. Secondary to these were the temporal variables of diel and survey timing, and the morphological characteristic of aggregation area. Lower numbers of aggregations were observed close to the herring spawning grounds but with higher acoustic energy than larger numbers of aggregations observed further from the spawning grounds but smaller in size and lower in acoustic energy. Most aggregations were in the lower portion of the water column, but those that were in the upper portion of the water column had higher acoustic energy. Consistently throughout the decade, 90% or more of herring aggregations were located within 40 nautical miles of their spawning grounds. The regression tree method provided valuable insight to the data series where it highlighted spatial and temporal patterns and was an effective way to quantitatively summarize relationships.

Spawning aggregation of the lanternfish Diaphus danae (family Myctophidae) in the north-western Coral Sea and associations with tuna aggregations

A spawning aggregation of the lanternfish Diaphus danae in the Coral Sea, that is fed on by spawning aggregations of yellowfin and bigeye tuna, has been sampled by mid-water trawling for the first time. Males and females in the aggregation occurred at a ratio of 23 to 1 and occupied two non-overlapping size classes (males 71.2–95.1-mm standard length (SL), females 99.0–121.4-mm SL). Hydrated oocytes with single oil droplets, which indicated imminent spawning, were in higher proportion in the first trawl (2134 hours to 2234 hours) than in the final trawl (0324 hours to 0424 hours) through the aggregation. Maximum estimated female D. danae fecundity (25 803) and gonadosomatic index (34.01) were higher than for any other lanternfish species recorded. Bigeye tuna (Thunnus obesus) individuals collected from aggregations were estimated to have 81–319 D. danae specimens in their stomachs. The annual Coral Sea D. danae aggregation is the only confirmed lanternfish spawning aggregation in Australian waters. The D. danae spawning aggregation provides a rich, transient foraging resource for spawning bigeye and yellowfin tuna in the Coral Sea, these being the only known tuna spawning aggregations in the Eastern Tuna and Billfish Fishery.

Relationship between mid-water trawling effort and catch composition uncertainty in two large lakes (Huron and Michigan) dominated by alosines, osmerids, and coregonines

Because it is not possible to identify species with echosounders alone, trawling is widely used as a method for collecting species and size composition data for allocating acoustic fish density estimates to species or size groups. In the Laurentian Great Lakes, data from midwater trawls are commonly used for such allocations. However, there are no rules for how much midwater trawling effort is required to adequately describe species and size composition of the pelagic fish communities in these lakes, so the balance between acoustic sampling effort and trawling effort has been unguided. We used midwater trawl data collected between 1986 and 2008 in lakes Michigan and Huron and a variety of analytical techniques to develop guidance for appropriate levels of trawl effort. We used multivariate regression trees and re-sampling techniques to i. identify factors that influence species and size composition of the pelagic fish communities in these lakes, ii. identify stratification schemes for the two lakes, iii. determine if there was a relationship between uncertainty in catch composition and the number of tows made, and iv. predict the number of tows required to reach desired uncertainty targets. We found that depth occupied by fish below the surface was the most influential explanatory variable. Catch composition varied between lakes at depths <38.5m below the surface, but not at depths ≥38.5m below the surface. Year, latitude, and bottom depth influenced catch composition in the near-surface waters of Lake Michigan, while only year was important for Lake Huron surface waters. There was an inverse relationship between RSE [relative standard error=100×(SE/mean)] and the number of tows made for the proportions of the different size and species groups. We found for the fifth (Lake Huron) and sixth (Lake Michigan) largest lakes in the world, 15–35 tows were adequate to achieve target RSEs (15% and 30%) for ubiquitous species, but rarer species required much higher, and at times, impractical effort levels to reach these targets.

Long‐Term Shifts in the Lateral Distribution of Age‐0 Striped Bass in the San Francisco Estuary

AbstractLike several other fishes in the pelagic community of the upper San Francisco Estuary, age‐0 striped bass Morone saxatilis have shown a major decline based on a midwater trawl sampling program that has been conducted for over 40 years. We hypothesized that the apparent decline in age‐0 striped bass might be partially attributable to a behavioral shift away from the channels sampled by the trawls. We found no evidence of an upstream–downstream shift in age‐0 distribution. Instead, age‐0 striped bass distribution remains closely associated with the low‐salinity zone of the estuary. However, the survey data suggest a substantial long‐term distribution shift away from channels and toward shoal areas. The hypothesis that young striped bass are undersampled by midwater trawls is supported by modeling of demographic patterns, which showed that the decline in numbers of age‐0 fish was not consistent with increasing trends in age‐1 fish. We hypothesize that reduced food availability in pelagic habitat is a major cause of apparent behavioral shifts by age‐0 striped bass and some native fishes. Nonetheless, the magnitude of the shift toward shoal habitat does not appear to fully account for the extreme decline in age‐0 striped bass abundance.

Temporal and spatial patterns in pelagic trawl fish catches in Lake Winnipeg

To better understand patterns of temporal and spatial variation of fish assemblages in offshore waters of Lake Winnipeg (Manitoba, Canada), midwater trawl tows were conducted near lakewide monitoring stations from 2002 to 2008. Trawl samples collected during spring, summer, and fall from the south basin, channel, and north basin were used to study effects of season and geographic region within the lake on species biomass estimates. Within each region, effect of trawl depth was explored. Greatest biomass in trawl catches was from the south basin. Of the most commonly caught species, across all seasons, greater biomass of emerald shiner (Notropis atherinoides) and cisco (Coregonus artedi) were found in the south basin and the channel, compared to the north basin. Biomass of the non-native rainbow smelt (Osmerus mordax), by contrast, was greater in the north basin compared to the south basin or the channel. Yellow perch (Perca flavescens) and introduced white bass (Morone chrysops) biomass varied temporally and spatially, with greatest biomass captured in the summer in the south basin. Estimated biomass of walleye (Sander vitreus) was greatest in the south basin, followed by the channel, and the north basin. Patterns in species distribution in Lake Winnipeg probably are influenced by a combination of factors, including species interactions and differing temperature and light conditions in the lake. As no lakewide pelagic trawl studies have been reported for this lake, these data form a baseline against which effects of changes, such as lake trophic state or establishment of non-native species, can be assessed.

Spatially explicit estimation of fish length histograms, with application to anchovy habitats in the Bay of Biscay

AbstractPetitgas, P., Doray, M., Massé, J., and Grellier, P. 2011. Spatially explicit estimation of fish length histograms, with application to anchovy habitats in the Bay of Biscay. – ICES Journal of Marine Science, 68: 2086–2095. Fish length frequency histograms from research surveys are of prime importance for identifying habitats of different life stages, as well as for stock assessment. However, no method has thus far been available for mapping these histograms as spatially varying curves. Here, a procedure is applied to map spatially connected curves, and detail is given on how it can be applied to map the length frequency histograms. At each sample location, a fish length frequency histogram is given as a vector of non-independent values. The histogram is first modelled as a polynomial expansion on the basis of orthogonal polynomials. Then, the polynomial coefficients are mapped by co-kriging, after fitting a model of co-regionalization. The length frequency map is finally derived by linearly combining maps of polynomial coefficients. An estimation variance associated with the map is also derived. Maps of anchovy length distributions are produced by applying the method to midwater trawl length data from the PELGAS acoustic surveys in the Bay of Biscay. This novel approach extends the application of kriging techniques to curves or functions, opening new perspectives for mapping more complex information than just the values of fish density.

Definition, dynamics and stability of métiers in the Irish otter trawl fleet

The Irish otter trawl fleet operates in a complex multi-species multi-gear fishery, spanning a wide geographic area, and involving around 275 trawlers. Factorial and clustering methods were applied to 2003 fishing trip data to define thirty-three métiers. Definitions were based on six trip characteristics taken from logbooks, namely: fishing gear, mesh size, vessel length, species composition, area, and month. Métiers exploiting demersal species or species groups are characterised by single vessel bottom otter trawls, typically with mesh sizes of 70 mm or more, operating year round. This includes nine Nephrops dominated métiers highlighting the importance of this species to the fleet. Many demersal métiers are characterised by groups of species, such as mixed whitefish or slope species. Métiers exploiting pelagic species are often focussed on single species, and are typically seasonal, mid-water trawling (often paired) with mesh sizes less than 70 mm. Pelagic métiers account for the majority of landings by over an order of magnitude in several cases. Demersal métiers account for the majority of fishing trips and effort, (primarily Nephrops métiers), and vessels (primarily mixed species métiers). The new métier definitions were found to be appropriate remained relevant despite declining fleet landings and effort between 2003 and 2006. Species compositions within these métiers have generally remained similar to the proportions defined in 2003. These robust métier definitions present opportunities to improve fisheries sampling, assessment and management. Although métiers pose complexity challenge for such applications they can be used the building blocks for appropriate management units.

Trophic connections in Lake Superior Part I: The offshore fish community

Detailed diet linkages within the offshore (> 80 m bathymetric depth) food web of Lake Superior are currently not well identified. We used analyses of fish stomach contents to create an empirically based food web model of the Lake Superior offshore fish community. Stomachs were collected seasonally (spring, summer, and fall) from nine offshore locations in 2005, using bottom and midwater trawls. In total, 2643 stomachs representing 12 fish species were examined. The predominant fish species collected were deepwater sculpin ( Myoxocephalus thompsonii), siscowet ( Salvelinus namaycush siscowet), kiyi ( Coregonus kiyi), and cisco ( Coregonus artedi). Mysis diluviana was the most common prey item, indicating that changes in Mysis abundance could have a profound impact on the entire offshore food web. Mysis was the primary diet item of deepwater sculpin (≥ 53% by mass) and kiyi (≥ 96% by mass) regardless of depth or season. The invasive Bythotrephes was an important diet component of the pelagic cisco in summer and fall. Deepwater sculpin were the primary diet item of siscowet (≥ 52% by mass), with coregonines appearing in the diet of larger (> 400 mm) siscowet. Non-metric multidimensional scaling analysis indicated that there were no statistically significant seasonal or site-specific differences in diets of deepwater sculpin, cisco, or kiyi. Site was the primary structuring factor in siscowet diets. Generally, in Lake Superior, the diet items of the dominant offshore species did not appear to be in danger from those types of major ecological shifts occurring in the lower Laurentian Great Lakes.

Population dynamics and production of Themisto gaudichaudii (Amphipoda, Hyperiidae) at South Georgia, Antarctica

The population dynamics, individual growth rates and productivity of the hyperiid amphipod Themisto gaudichaudii were studied in Cumberland Bay, a fjord on the north-eastern coast of South Georgia. The study involved the weekly sampling of the population and its potential food with a Rectangular Midwater Trawl (RMT1) at weekly intervals between May 2005 and October 2006. Offshore samples were taken the previous austral summer using an RMT8 net. T. gaudichaudii produced one main cohort in the spring (September/October) and a secondary weaker cohort in the late summer (January/February). The main cohort appeared at the time of phytoplankton increase, about 3 weeks before an increase in mesozooplankton. Larger individuals (20–25 mm) resided offshore but not within the Bay during the main recruitment period. T. gaudichaudii grew at an average rate of 0.1 mm d −1 during the first four months after release and then by 0.07 mm d −1 for the subsequent four months. At these rates, it is possible that the main cohort reaches reproductive size in time to spawn the autumn cohort, although the relative weakness of the autumn cohort suggests that few manage this. T. gaudichaudii biomass reached its highest levels in November at a mean value of 5.4 mg DW m −3 while daily production was also highest in November at a mean value of 0.17 mg DW m −3 d −1. The P/B d −1 ratio varied between 0.03 d −1 in the summer to below 0.01 d −1 in the winter. Daily mortality rate was initially 0.12 d −1 during the first two months post-recruitment, falling to 0.03 d −1 for the remainder of the year. Integrating production over the year gave a value of 10.04 mg DW y −1, equivalent to 3.62 mg C m −3 y −1, making T. gaudichaudii a relatively productive component of the pelagic community. During the summer, the species was estimated to consume around 1–2% of available mesozooplankton biomass and a mean of 15% of mesozooplankton daily productivity. T. gaudichaudii may have a controlling influence on mesozooplankton dynamics as well as on the recruitment of the larval stages of important commercially fished species such as Antarctic krill ( Euphausia superba) and Mackerel icefish ( Champsocephalus gunnari).

DEPM-based spawning biomass of Emmelichthys nitidus (Emmelichthyidae) to underpin a developing mid-water trawl fishery in south-eastern Australia

Spawning biomass of redbait, Emmelichthys nitidus (Emmelichthtyidae), was estimated using the daily egg production method (DEPM) based on egg and adult surveys conducted simultaneously off eastern Tasmania during October 2005 and 2006. Concurrent studies had confirmed that this mid-water, schooling species met all necessary requirements for DEPM-based biomass estimation, including (a) asynchronous oocyte development with release of pelagic eggs in batches; (b) spawned eggs could be assigned ages using a temperature-dependent incubation model; and (c) egg abundances follow the typical exponential decay model. Main spawning areas were identified between north-eastern Bass Strait (38.8°S) and south of the Tasman Peninsula (43.5°S) in 2005 (13220 km 2), and between Cape Barren Is. (40.5°S) and the same southern boundary in 2006 (8695 km 2). Daily egg production ( P 0) was estimated by applying two statistical estimation methods to the egg abundance-at-age data, namely the traditional least squares non-linear regression (NLS) and a generalized linear model (GLM). Results indicated that the latter technique provided a better fit, resulting in improved CVs and AIC statistics over the NLS. The GLM-derived average P 0 was estimated at 4.04 eggs/0.05 m 2/day both in 2005 and 2006. Spawning biomass (CV) was ∼87000 t (0.37) in 2005 and ∼50800 t (0.21) in 2006, with the lower 2006 biomass largely due to a smaller spawning area and higher sex ratio. Estimates are likely to be negatively biased since spawning of E. nitidus probably extends as far north as southern New South Wales (35.0°S). In the absence of comparable studies on other emmelichthyids, P 0 and instantaneous egg mortality estimates are compared to those of clupeoid species previously subjected to DEPM, and the method discussed in terms of its suitability to this species in support of a developing mid-water trawl fishery in south-eastern Australia.

Sexual dimorphism in body shape of Antarctic krill ( Euphausia superba) and its influence on target strength

Sexual dimorphism in the body shape of Antarctic krill ( Euphausia superba) was investigated and its influence on target strength (TS) was clarified using a theoretical scattering model. The TS which is used to convert acoustic backscatter to krill density was also presented. Body shape data were obtained from 456 specimens (54 juveniles, 200 males, and 202 females) collected off Adélie Land using a Rectangular Midwater Trawl. The sexual dimorphism manifested as a swollen cephalothorax in female krill with body lengths exceeding 40 mm. The TS of female krill was higher than those of male krill at low frequencies, even when body lengths were the same. This is because of the Rayleigh scattering region and the transition region to the geometric scattering region. The influence of the sexual dimorphism on the TS was small at frequencies exceeding 70 kHz, which are close to the geometric scattering region. The regression curve derived from the predicted TS of 456 specimens was in reasonable agreement with the measured TS in other previous studies, and the regression curve could be applied to the acoustic surveys of Antarctic krill.

The use and efficacy of acoustic survey methods as applied to a mixed aquatic community: The deep scattering layer of the California Current.

Concurrent acoustic and trawl data were collected in 2008 near Point Conception to compare the abundance estimates of mesopelagic micronekton obtained with these two methods. Acoustic data were obtained with a calibrated EK‐60 equipped with four frequencies (38, 70, 120, and 200 kHz). Animals were collected with a midwater trawl. Target strength (TS) models were created of the animals collected by the trawl, and animals were assigned to acoustic groups based on the frequency spectrum of their modeled TS. The mean spectra of the acoustic groups were used with non‐negative least squares inverse methods to estimate abundance from the multi‐frequency volume scattering. The resulting acoustic abundances of each group were compared to the trawl catches, and capture efficiency of the net was estimated. The accuracy of the inverse method was tested with a Monte Carlo analysis of an artificial echogram. The accuracy of the forward modeling was tested via comparison to single targets recorded by the EK‐60. [Work supported by NASA, CCE‐LTER, and The Moore Foundation.]

Influence of biophysical coupling on age‐0 pollock survey results.

Age‐0 walleye pollock relative abundance and distribution in the eastern Bering Sea has been characterized with data from a surface trawl survey [Bering Aleutian Salmon International Survey (BASIS)]. Based on surface trawl catches since 2003, age‐0 pollock abundances appeared to be highest during climatic warm years and lowest in cold years, with predictable distributions based on water column stratification. In 2008–2010, all cold years, acoustics, and midwater trawling were added to the BASIS survey. Acoustics confirmed that low numbers of age‐0 pollock were found in the surface waters, but revealed the presence of previously unsampled, large (over 50 m high, several km long) aggregations in deep (greater than 100‐m bottom depth) water. Acoustic and midwater trawling results suggest that the deep biomass may exceed that found in surface waters. Relationships are now being evaluated between oceanographic characteristics (water column stability, extent of the bottom cold pool) and biomass of age‐0 pollock in surface and deep aggregations. Clarifying these biophysical linkages is important for evaluating potential gear‐specific biases in survey data and for informing survey design decisions under changing climate conditions.

Acoustic‐trawl surveys to assess walleye pollock in Alaska: challenges faced and progress made.

Large‐scale acoustic‐trawl surveys have been regularly conducted for over 3 decades by researchers at the NOAA, Alaska Fisheries Science Center (AFSC) to assess walleye pollock (Theragra chalcogramma) in Alaska. Research within the AFSC acoustics program (MACE) has addressed several of the challenges associated with both the acoustics and trawling aspects of these surveys. Acoustic issues include species classification of the acoustic data, fish avoidance to survey vessels, and in situ target strength measurements. Large trawls typically form an integral part of acoustic surveys to confirm the species identity of backscatter and to collect other biological information needed to convert the echo integral into numbers and weight of animals per unit area. MACE staff have also made significant progress in evaluating the selectivity of the large midwater trawls used during their acoustic surveys as well as in developing other direct sampling tools to help interpret the acoustic backscatter data. This presentation will highlight these and other recent advanced technologies and methods that are being developed and used by the MACE program to improve acoustic‐trawl surveys in Alaska and elsewhere.

Characterizing and monitoring marine nekton at Puget Sound’s renewable energy site.

Marine hydrokinetic energy sites have the potential to impact distributions, dynamics, and abundances of macroinvertebrates, fish, and marine mammals (i.e., nekton). Potential impacts include changes in aggregation, avoidance, and occurrences of strikes or impingements. Understanding potential impacts requires knowledge of species‐specific distributions over relevant spatial and temporal scales. Configurations and integration of technologies capable of providing images and data are not well‐established, and the application of monitoring technologies is complicated by extreme flows at marine hydrokinetic energy sites. An echosounder, multibeam sonar, and acoustic camera will be used to detect, categorize, and enumerate nekton at a proposed renewable energy site in northern Admiralty Inlet, Puget Sound, WA. This is the site selected by Snohomish Public Utility District for the deployment of two OpenHydro turbines. Data from stationary instrument deployments will be compared to data from a mobile acoustic and midwater trawling survey to determine how well each technology captures spatiotemporal variation in nekon density distributions. Results of the design, deployment, retrieval, and analysis of data from these three instrument classes will be used to formulate recommendations for instrument choice, configuration, and the characterization and monitoring of pelagic nekton at any renewable energy site.

Partial diel vertical migrations in pelagic fish

1. Field studies on diel vertical migration (DVM) usually report uniform behaviour with population-wide ascents and descents during crepuscular periods. This contrasts partial seasonal migrations of many animal populations, where individuals choose either the resident or the migrant strategy depending on population density, feeding opportunity and predation risk in the resident and migrant habitats. 2. We tested whether DVM of freshwater zooplanktivorous fish (Coregonus spp.) resembles partial migrations. Twenty-eight hydroacoustic surveys were performed in the deep Lake Stechlin (Germany) between 2000 and 2010, with samplings encompassing all months between March and December. Zooplankton samples were simultaneously taken in epilimnetic and hypolimnetic layers. Fish obtained from depth-stratified samplings by a midwater trawl were used to test for individual differences between residents and migrants. 3. We show for the first time that DVMs of freshwater fish resemble patterns of partial migrations often found in seasonal environments. Across all samplings, 7-33% of fish did not ascend at dusk, but exhibited the resident strategy. The proportion or residents increased at low zooplankton feeding rates in the daytime habitat and during months when the temperature difference between daytime and night-time habitats was minor. 4. Slightly larger size and higher caloric density of migrants over residents in one of the coexisting Coregonus species suggested that individual differences contributed to the migration strategy performed. However, these results were based on one sample only, and extrapolation to the entire data set is not possible. 5. Our results are indirect evidence that the balance between migrants and residents may primarily depend on the trade-off between feeding gains and metabolic and predation costs of migration. However, the results also suggest that the global fitness consequences for the resident and migrant strategies may not be identical, rendering the importance of individual traits in the 'decision to migrate' likely.