Spatial abundance patterns and recruitment of a virus‐affected commercial mollusc fishery

Infectious pathogens figure prominently among those factors threatening marine wildlife. Mass mortality events caused by pathogens can fundamentally alter the structure of wild fish stocks and depress recruitment rates and yield. In the most severe instances, this can precipitate stock collapses resulting in dramatic economic losses to once valuable commercial fisheries. An outbreak of a herpes‐like virus among commercially fished abalone populations in the south‐west fishery of Victoria, Australia, during 2006–2007, has been associated with high mortality rates among all cohorts. Long‐term records from fishery‐independent surveys of blacklip abalone Haliotis rubra (Leach) enabled abundance from pre‐ and post‐viral periods to be analysed to estimate stock density and biomass. The spatial distribution of abundance in relation to physical habitat variables derived from high‐resolution bathymetric LiDAR data was investigated. Significant differences were observed in both measures between pre‐ and post‐viral periods. Although there was some limited evidence of gradual stock improvement in recent years, disease‐affected reefs have remained below productivity rates prior to the disease outbreak suggesting a reduction in larval availability or settlement success. This was corroborated by trends in sublegal sized blacklip abalone abundance that has yet to show substantial recovery post‐disease. Abundance data were modelled as a function of habitat variables using a generalised additive model (GAM) and indicated that high abundance was associated with complex reef structures of coastal waters (<15 m). This study highlights the importance of long‐term surveys to understand abalone recovery following mass mortality and the links between stock abundance and seafloor variability.     

The enhancement of abalone stocks: lessons from Japanese case studies

The dramatic declines in abalone Haliotis spp. fishery production have been documented all over the world. Release of hatchery‐reared juveniles into natural habitats has been considered as one measure to sustain and/or augment the current fishery production of abalone, as well as to restore collapsed abalone stocks. However, attempts at abalone release programmes have only been undertaken at experimental scales, except for Japan, where large‐scale stock enhancement programmes for abalone have been undertaken since late 1960s. To evaluate the potential of stock enhancement for abalone, we analysed the release surveys of 13 case studies in Japan in terms of the overall recapture rate (number of recaptures through a lifetime/number of juveniles released), yield per release (YPR, yields from released individuals), the economic efficiency of releases (ratio of income from recaptured abalone to release cost) for each release year, and the contribution of hatchery releases to total catches for each fishing year. The average estimates for overall recapture rates (0.014–0.238) and YPR (3.1–60.3 g/individual) varied between locations and release years. The economic efficiency was estimated at 0.4–6.2. The released abalone contributed 6.9–83.5% to total catches. Hatchery releases could augment total production at some locations, but the success of release programmes would be limited by the carrying capacity at release areas, because density‐dependent mortality occurred following releases in some cases. Throughout Japan, the annual catch of abalone has continuously declined from ～6500 t in 1970 to ～2000 t in the mid‐1990s, despite the increase in the number of hatchery releases. Based on the estimates for YPR, the magnitude of the abalone releases on a national scale has not been sufficiently large to sustain the total production of Japanese abalone, which has primarily fluctuated according to the abundances of wild populations. Our results suggest that releases should be targeted at local populations in regions where stock enhancement is predicted to have the greatest chance of success, and the magnitude of releases should be considered carefully and determined for each region by taking the local carrying capacity into account. We also address the future prospects of abalone stock enhancement.     

Historical ecology can inform restoration site selection: the case of black abalone (Haliotis cracherodii) along California's Channel Islands

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Incorporating cannibalism into an age-structured model for the Chilean hake

We incorporated predation equations from the multispecies virtual population analysis model MSVPA into an age-structured model for the Chilean hake (Merluccius gayi gayi) to estimate cannibalism. Two models, model I with constant natural mortality and the MSM, were fitted to the total annual catch, spawning biomass from acoustic surveys and length composition data from fishery and acoustic surveys. Model I fitted the data better than MSM. The majority of the MSM estimates of adult population and spawning biomass were larger than the model I estimates; probably due to the choice of residual mortality M1. High estimates of predation mortality were observed for age-0 hake. In spite of a decreasing fishing mortality, the spawning biomass decreased in the last years. Preliminary MSM results suggest that this might be due to an increase in cannibalism. A sensitivity analysis suggested all response variables were not sensible to the “other food” parameter but sensible to M1 and the predator annual ration. MSM is a promising approach that introduces the predation mortality equations into a statistical framework, allowing the incorporation of the uncertainty in the estimation of the parameters and the use of standard statistical tools in a multispecies context. This approach will contribute to provide useful information on the indirect effects of fishing on non-target species to fisheries managers.     

Acoustical-optical assessment of Pacific herring and their predator assemblage in Prince William Sound,Alaska

The Pacific herring Clupea pallasi population in Prince William Sound (PWS), Alaska, is both a valuable commercial resource and an important forage species for marine fish and wildlife. Historically, the herring were managed by a combination of age-structured models and egg deposition estimates. When these methods predicted a large return for spring 1993 that failed to materialize, we began surveying with echointegration–purse seine methods. After a decade of acoustic surveys, we show the new approach yields highly precise biomass estimates, which are consistent with historical measures of the miles of beach spawning. When compared, we show the traditional methods overestimated stock biomass, which resulted in harvest rates approaching 40%. In contrast, the acoustic methods are most likely to underestimate biomass. Since the acoustic estimates can be quickly obtained, we recommend their use to set harvest quotas for the fishery in the spring just prior to harvest. The shift from the traditional preseason to inseason management practices for herring in PWS is consistent with the Precautionary Principle by the fact that protection of the spawning population does not rely on the ability of science to predict how the population is changing. Furthermore, synoptic infrared measurements on our night-time acoustic surveys revealed herring to be the most important winter forage to marine birds and wildlife in PWS, including the endangered Steller sea lion Eumetopias jubatus. Given the importance of forage to marine birds and wildlife in the North Pacific during the extended winter conditions (October–March), the implementation of inseason management for herring using echointegration–purse seine techniques may be the most effective method to restore depressed populations of marine birds and mammals in the North Pacific.     

Do predators,handling stress or field acclimation periods influence the survivorship of hatchery‐reared abalone Haliotis kamtschatkana outplanted into natural habitats?

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Changes in yellow perch length frequencies and sex ratios following closure of the commercial fishery and reduction in sport bag limits in southern Lake Michigan

Abstract  The closure of the Indiana commercial fishery in Lake Michigan in 1997 provided the opportunity to examine the effects of commercial exploitation on the yellow perch, Perca flavescens (Mitchill), population. Data from annual gillnet and trawl sampling, recreational angler creel surveys and commercial fishing harvest were used to determine whether changes in length–frequency distributions and sex ratios were altered following closure. The proportions of large, adult yellow perch were significantly greater after the closure of the commercial fishery. Overexploitation of fish >200 mm by commercial harvest likely truncated the size distribution, but recreational harvest was comparatively small and did not appear to be a factor influencing size distributions. This post-closure period also had a greater proportion of females in trawl catches, and the mean length of harvested fish increased in the recreational fishery. These findings suggest commercial exploitation influenced yellow perch population dynamics in far southern Lake Michigan.     

Defining a Safe Operating Space for inland recreational fisheries

The Safe Operating Space (SOS) of a recreational fishery is the multidimensional region defined by levels of harvest, angler effort, habitat, predation and other factors in which the fishery is sustainable into the future. SOS boundaries exhibit trade‐offs such that decreases in harvest can compensate to some degree for losses of habitat, increases in predation and increasing value of fishing time to anglers. Conversely, high levels of harvest can be sustained if habitat is intact, predation is low, and value of fishing effort is moderate. The SOS approach recognizes limits in several dimensions: at overly high levels of harvest, habitat loss, predation, or value of fishing effort, the stock falls to a low equilibrium biomass. Recreational fisheries managers can influence harvest and perhaps predation, but they must cope with trends that are beyond their control such as changes in climate, loss of aquatic habitat or social factors that affect the value of fishing effort for anglers. The SOS illustrates opportunities to manage harvest or predation to maintain quality fisheries in the presence of trends in climate, social preferences or other factors that are not manageable.     

Variation in productivity of a key snapper,Chrysophrys auratus,fishery related to recruitment and fleet dynamics

Abstract Temporal variation in production of the Chrysophrys auratus (Forster) fishery of Northern Spencer Gulf, South Australia, between 1984 and 2009 was investigated using commercial fishery statistics and estimates of age and size structures from market sampling. Fishery production was divisible into two periods, i.e. from 1984 to early 2000s and the middle to late 2000s. During the former, cycles in production were relatable to variable fishable biomass. Age structures that displayed considerable differences in age class strength reflected interannual variation in juvenile recruitment, which resulted in high variability in the strength of year classes entering the fishery. Periodic strong year classes were the major influence on fishable biomass as they passed through the population over numerous years. For the latter period, fishery statistics were conflicting as catches and effort were relatively low, whilst the estimates of catch per unit effort (CPUE) were consistently high. These statistics reflect a considerable change to the structure of the fishery that involved a reduction in number of fishers and fishing effort. The fishery statistics may indicate hyperstability, with CPUE estimates remaining high despite declining biomass because of the aggregative behaviour of the fish and the efficiency of remaining fishers.     

Population age structure and mortality rates of jackass morwong (Nemadactylus macropterus) in Eastern Bass Strait,Australia

The catch-curve method was used to estimate the instantaneous rate of total mortality (Z) of jackass morwong (Nemadactylus macropterus) in south-east Australian waters. Age-frequency distributions were obtained for fish collected from eastern Bass Strait during the 3 years. March 1982–February 1985. Fish were obtained from: (1) demersal research fishing surveys stratified by depth; (2) commercial landings from eastern Bass Strait; (3) commercial landings from the adjacent New South Wales fishery.Jackass morwong were found in water 65–299-m deep, and almost half the biomass was in water 90–179-m deep. Our estimate of the total biomass (from the survey results) of the part of the population in eastern Bass Strait was 15 700 t. The oldest male morwong caught was 11 years old, the oldest female 13 years old. From the survey data, total mortality (Z) was estimated as 0.45 for males, which is equivalent to an annual survival rate (S) of 0.64, and as 0.47 for females, equivalent to an S of 0.63. There was substantial annual variation in estimates of Z. For the survey results, the annual mortality rate estimated for males was significantly different from that for females.The instantaneous rate of fishing mortality (F) estimated from population biomass and mean commercial landings was very low (about 0.024; sensitivity analysis indicated a possible range of 0.012–0.072), and indicated that about 2% (by number; range 1–6%) of the part of the population in eastern Bass Strait was harvested annually.Z values estimated from samples of commercial landings were consistently larger than Z estimated from the survey samples. Using estimates of Z from commercial landings data resulted in high values of M (the instantaneous rate of natural mortality), which were inconsistent with the longevity of the species and the proportion of older fish found in the population. We believe this to be due to bias in the way commercial landings sample the population, and to be an inherent bias in the use of age-frequency data from commercial landings unless some correction for size-selectivity due to commercial fishing gear and fishing behaviour can be made.     

Identifying historical baseline at the onset of exploitation to improve understanding of fishing impacts

1. Marine communities have long been impacted by human activities, but the quantification of human‐driven changes often relies on recent data. This is because historical data on fish populations are lacking and are challenging to include in contemporary stock and ecological assessments. As a result, the impacts of early fishing pressure on marine communities are generally poorly documented worldwide.
2. Marine communities of Southeast Australia have a relatively short history of exploitation compared with other temperate systems and were sampled before and after the onset of commercial fishing. As such, they provide a rare opportunity to identify historical baselines and to understand ecological changes after the onset of commercial exploitation.
3. This study compares survey data collected around Tasmania, Southeast Australia, in 1909–1910 with data from the 1980s. The period considered precedes the establishment of a trawl fishery in Southeast Australia in 1915, of other important commercial fisheries in Tasmania, and of a fisheries data collection programme in 1984. Nominal catch rates are used to examine changes across all families of demersal fish recorded in catches and generalized linear models are used to estimate and compare standardized indices of abundance between the 1909–1910 and 1980s data for key commercial families.
4. Results show significant, and thus far unreported, fishing‐induced changes in marine communities after the establishment of commercial fishing in the region. Changes mostly relate to shifts in catch composition and steep declines in the abundance of the main commercial families.
5. This study illustrates a method for analysing low‐quality historical catch data and provides estimates of pre‐commercial fishing abundance that can be included in stock and ecological assessments. More broadly, this study demonstrates the significant role of early fishing in shaping marine communities and increases our understanding about general patterns of exploitation that have been difficult to identify with longer but less detailed fishing histories.

     

Does marine fish farming affect local small‐scale fishery catches? A case study in the NW Mediterranean Sea

Fish farms have been shown to aggregate large numbers of wild fish in their surroundings. Although little is known about how this affects the local fishery, two hypotheses have been put forward; a trapping and a protecting effect on wild fish. This study provides the first monitoring of commercial and recreational fishing activity at a fish farm. We assessed the effect on the small‐scale fishery analysing the differences between fishing in farm proximity and away from the farm (in terms of catch rate, income and catch composition). Moreover, we estimated the biomass removal from fishing activities by farm employees inside the farm. The study was conducted from January 2011 to June 2012 at a Gilthead seabream farm in the NW Mediterranean. The findings revealed a relevant amount of commercial and recreational fishing effort in farm proximity. Yet, the results showed no benefit or difference of fishing in close proximity to the farm compared with areas away from the farm. Thus, we conclude that farm‐aggregated fish are protected from the commercial fleet by the farm leasehold area, but remain vulnerable to hidden fishing practices inside the farm, where farm employees harvest more than 4 t of wild fish annually.     

Multi-decadal scale variability in the eastern Baltic cod fishery 1550–1860—Evidence and causes

Identification of periods of high and low cod production, and the reasons for these periods, can increase understanding of variability in populations and ecosystems. In this study we investigate the multi-decadal and multi-century scale variations in the cod population in the eastern Baltic Sea (ICES Subdivisions 25–32). Analytically derived estimates of biomass are available since 1966. These estimates show that biomass increased in the late 1970s–early 1980s, but decreased nearly 10-fold until the early 1990s and is still well below the long-term average. Prior to 1966 the biomass of cod is unknown, as is the relative role of fishing, climate variability/regimes, eutrophication and reduction of marine mammal predator populations. We have begun to investigate whether historical fisheries information (landings, effort, distribution) from before the 1880s is available in Baltic archives and museums, and to what extent this information can be used to interpret variations in this population. We have located fisheries data for different parts of the Baltic for different time periods since the 1550s and have interpreted the findings using current process knowledge of oceanographic mechanisms affecting cod reproduction and ecology in the Baltic Sea. The recovered data show that the Baltic ecosystem was able to support modest-large cod populations even though it was oligotrophic and contained large populations of cod predators (e.g., marine mammals). Current ecosystem management policy in the Baltic as developed and implemented by organisations such as the International Council for the Exploration of the Sea (ICES), the Baltic Marine Environment Protection Commission (HELCOM), the nine coastal countries and the European Union includes recovery of the cod population, a reduction in nutrient loading and measures to promote recovery of seal and harbour porpoise populations. If these policies are successful, the role of predatory fish in the future Baltic could again be substantial and comparable to that which we show existed 450 years ago. However, such a scenario will also require a major reduction in cod fishing mortality and suitable hydrographic conditions which promote successful cod reproduction. Historical ecology investigations in the Baltic can contribute to scientifically based fishery and ecosystem management and recovery plans.     

Optimising harvest strategies in a multi‐species bivalve fishery

Abstract Optimising harvests from fishery resources requires appropriate data. In this study, fishery‐independent survey and size‐at‐first maturity (L₅₀) data were used to assess the suitability of current minimum legal sizes (MLS) and total allowable commercial catches (TACC) in the South Australian, mixed‐species, mud‐cockle (Katelysia spp.) fishery. Estimates of L₅₀ suggested the MLS was conservative in one fishing zone (Coffin Bay), but appropriate elsewhere. Harvestable‐biomass estimates (HB) demonstrated that TACCs were excessive in the Port River (41% of HB), suitable in Coffin Bay (10% of HB) and precautionary in the West Coast (1.5% of HB) fishing zones. Consequently, the MLS was decreased by 5 mm shell length in Coffin Bay and the TACCs for the Port River (reduced by 80%) and West Coast (increased by 40%) amended. This study demonstrated that harvest strategies in mixed‐species fisheries can be optimised by explicitly considering data on species composition, abundance and population biology.     

Are hatchery‐reared abalone naïve of predators? Comparing the behaviours of wild and hatchery‐reared northern abalone,Haliotis kamtschatkana (Jonas, 1845)

Abalone populations have declined worldwide, generating interest in enhancement using hatchery‐reared individuals. In many cases, such restoration efforts have met with limited success due to high predator‐induced mortality rates. Furthermore, the mortality rates of outplanted hatchery abalone are often considerably higher than for wild individuals. This study uses northern abalone (Haliotis kamtschatkana) as a case study to determine whether hatchery‐reared abalone behave differently than their wild counterparts. In the field, outplanted hatchery‐reared abalone were significantly less responsive than wild abalone, in terms of number of abalone responding and intensity of response, to nearby movement and to physical contact with an inert probe. Also, when encountering a cue to which all abalone responded (a seastar predator), hatchery‐reared individuals remained subdued. Anti‐predator behavioural deficits in hatchery‐reared abalone were more pronounced in 4‐year‐old individuals than in 1‐year‐old individuals, suggesting an influence of either age or amount of time spent in the hatchery environment. These behavioural differences are expected to increase the vulnerability of hatchery‐reared abalone to predators, and are likely a major cause of their elevated predator‐induced mortality when outplanted.     

Fishing efficiency of competitive largemouth bass tournament anglers has increased since early 21st century

Tournament fishing has risen in popularity over the last half a century. As such, social and financial incentives combined with technological advancements are expected to drive changes in angler's capacity to exploit tournament‐eligible fish stocks, as has been observed in commercial fisheries. The aim of this study was to quantify temporal trends in angler efficiency and their ability to exploit a given fish stock relative to effort in largemouth bass fishing tournaments. A collective analysis across seven Illinois reservoirs comparing change through time in angler catch rates and relative population abundances indicated that angler efficiency has generally improved through time. For the decade from 2005 to 2015, a greater than threefold increase in the efficiency of anglers to exploit a static population of largemouth bass was estimated. Anglers have become more efficient at exploiting populations, which is likely to influence management decisions in the future, particularly in harvest‐orientated fisheries and those reliant upon fishery‐dependent surveys.     

Fishery benefits from behavioural modification of fishes in periodically harvested fisheries closures

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Microsatellite DNA analysis of overwintering bull trout (Salvelinus confluentus) and its implications for harvest regulation and habitat management

A mixed-stock fishery occurs when multiple populations of a fish species are exploited together in a common area where they aggregate outside the breeding season (e.g. for feeding or overwintering), and the aggregation is known as a mixture. Recreational fishing often exploits such mixtures, and estimating the proportional contributions of populations to fisheries promotes more sustainable resource use. Ten DNA microsatellites were assayed in a mixture of bull trout Salvelinus confluentus Suckley overwintering in the Nechako River, upper Fraser River, British Columbia, and in baseline population samples from 14 tributaries putatively contributing to the overwintering mixture. A DNA microsatellite-based mixed-stock fishery analysis suggested that five populations together contributed 0.80 to the mixture. Most of the errors associated with the mixture estimates were attributable to uncertainty in baseline allele frequencies. Radiotracking data confirmed that tributary populations contributing to the mixture estimated by genetic analysis also contained individuals that moved between spawning tributaries and overwintering sites. The results better resolve habitat use by potadromous bull trout in the upper Fraser River and, in combination with assessments of baseline population-specific spawning abundances and productivity, will better inform a decision of whether or not allowing some harvest within the current catch-and-release fishery is biologically sustainable.     

Feasibility of Offshore Co‐culture of Abalone,Haliotis discus hannai Ino,and Sea Cucumber,Apostichopus japonicus,in a Temperate Zone

Feasibility of offshore co‐culture of abalone and sea cucumber was investigated in Northern China. Survival and growth of abalone, Haliotis discus hannai Ino, and sea cucumber, Apostichopus japonicus, co‐cultured in abalone cages from suspended longlines, in the offshore area, were examined. Abalone and sea cucumbers were co‐cultured at density ratios of both 3:1 and 6:1 for 1 yr. Abalone were fed with fresh kelp and no additional feed was given to sea cucumbers. Survival of abalone and sea cucumber was 100% for all treatments. Abalone and sea cucumber grew well; the body weight (BW) of abalone and sea cucumber was nearly doubled and had reached a commercial size. There were no significant differences in the growth rates for both abalone and sea cucumber between the two density treatments. The specific growth rate of BW of abalone (SGRbw) was highest in June, with a value of 0.536%/d. Growth rate of sea cucumber (SGRsc) was highest in December, reached 1.84%/d, with an annual average SGRsc of 0.182%/d. Results suggested that the offshore co‐culture of abalone and sea cucumber was feasible offshore. The co‐culture of abalone with sea cucumbers may provide an additional valuable crop without additional financial input.     

Kittiwake breeding success in the southern North Sea correlates with prior sandeel fishing mortality

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