Variation In Catch Research Articles

Evaluation de la stabulation comme moyen de protection des bovins des piqûres de <em>Culicoides</em>, vecteurs de la fièvre catarrhale ovine

Light-trap catches of Culicoides biting midges were compared inside and outside animal housing, in the presence and in the absence of cattle. A three-time replicated 4x4 Latin square design was used at four farms in Bala, North Wales, over 12 nights in May and June; the experiment was repeated in October. In the two studies, over 70,000 and over 4500 Culicoides were trapped of which 93% and 86%, respectively, belonged to C. Obsoletus complex. Across the four farms, in May and June, the presence of cattle increased catches of C. obsoletus by 2.3 times, and outside traps caught them 6.5 times more than inside traps. Similar patterns were apparent in October, but the difference between inside and outside catches was reduced. Catches were strongly correlated with minimum temperature and maximum wind speed, and these two variables explained a large amount of night-to-night variation in catch. Outside catches were reduced to a greater extent than inside catches by colder minimum tem­peratures and stronger maximum wind speeds. These conditions occurred more frequently in October than in May and June, thereby suppressing the outside catches more than the inside catches, and reducing the apparent degree of exophily of C. obsoletus in autumn. The results suggest that the risk of animals receiving bites from C. obsoletus is reduced by housing at both times of year, but the benefit would be greatest on warm, still nights when outside catches are at their greatest.

Short-term climate variability and the commercial barramundi (Lates calcarifer) fishery of north-east Queensland, Australia

The sustainable productivity of estuarine fisheries worldwide is threatened by over-fishing, habitat destruction and water impoundment. In some cases, the natural variability of freshwater inputs has been shown to affect catch when low flows reduce nutrient input and inundated nursery habitats. Historically, the annual commercial catch of barramundi (Lates calcarifer) in Queensland has been highly variable for reasons not fully understood. In conjunction with a life-cycle model, statistical analyses of climate variables and barramundi catch data from the Princess Charlotte Bay area identified several significant relationships. Warm sea surface temperatures, high rainfall, increased freshwater flow and low evaporation (all measures of an extensive and productive nursery habitat) were significantly correlated with barramundi catch 2 years later and suggest that young barramundi survival is enhanced under these conditions. Catchability was significantly increased with high freshwater flow and rainfall events in the year of catch. A forward stepwise ridge regression model that included a measure of rainfall and evaporation 2 years before catch explained 62% of the variance in catch adjusted for effort. It is recommended that the impact of climate variability be considered in the management of wild barramundi stocks and possibly other species not yet examined.

Annual and Monthly Variation in Species Composition and Catches of Fishes from the Tabusintac River Estuary in the Southern Gulf of St. Lawrence

A total of 7130 fish from 13 taxa were collected during summer and autumn, 2002-2005 using box traps at two sites in the Tabusintac River Estuary, New Brunswick, Canada. The objective was to determine seasonal change in species composition and catches, and to access change in annual returns of Atlantic Salmon, an important recreational fishery during autumn on the Tabusintac, Miramichi and neighboring rivers. Taxa richness and composition varied annually and from trap to trap. Richness ranged from 8-10 taxa annually and catches were usually highest in the trap closest to the ocean despite its slightly smaller size. Taxa contributing greater than one percent of the total catch during the four years of sampling included six diadromous taxa: Blueback Herring and Alewife (41.65%), Striped Bass (21.54%), Atlantic Salmon (3.98%), Tomcod (2.95%), Brook Trout (1.05%), American Eel (5.27%); one freshwater species, White Sucker (11.96%); one marine species, Winter Flounder and one resident estuarine species, Smooth Flounder which together account for 11.19% of the total catch. The remaining five species (American Smelt, Northern Pipefish, White Perch, Cunner, Sea Lamprey) contributed 0.39% of the total catch. The fish fauna 14-15 km up the Tabusintac River was not species rich, due in large part to the limited sampling time each year and due to fishing with just one sampling gear, a stationary box trap. Additional species would have been collected if smaller mesh sampling gear were used. Comparisons were made with other studies and the phenology of the dominant species on the Tabusintac and Miramichi River Estuaries during the ice free season is established. Most catches appear to be related to pre- or post-spawning movements of anadromous fishes.

Summing Up - Symposium on Reproductive and Recruitment Processes of Exploited Marine Fish Stocks

Variability in recruitment results in variability in catch from year-to-year, which is a problem for fishermen, for the processing industry and for fisheries managers. It is commonplace to read how large the interannual variability in recruitment can be and how difficult it is to determine the causes. However for an ecologist, the astonishing feature of recruitment variability is how small it is, given that fish produce thousands or millions of eggs each and that survival is extremely unlikely and uncertain. Among the pelagic and demersal stocks assessed in the NE Atlantic the smallest year-class is generally within a factor of 10 of the biggest. It is not infrequent for interannual recruitment variability to be less than the variability of the spawning biomass of the stock (Brander, 2003). How is the variability damped out? What are the compensatory mechanisms? Density dependence is necessary for populations to remain within feasible bounds (Reddingius, 1971) but as Ed told us “A little density dependence, especially in the relatively long juvenile stage, can regulate recruitment”. Density dependence should not be invoked without good evidence in relation to processes like adult growth, however appealing it is to biologists or convenient for modellers trying to keep their models stable. Fisheries assessment and management, at least in the North Atlantic, has focussed on year-to-year tracking of stock biomass in order to set annual catch limits. Questions about the role of recruitment were therefore also principally directed at annual prediction and the understanding of processes needed in order to do this. The biological book-keeping was done using sequential population analysis which gives estimates of both annual recruitment and spawning stock biomass (SSB). The latter is assumed to represent spawning output of the stock, but as many of the papers in this symposium show, fecundity, maturity, spawning frequency and egg quality, are not constant. The relevance of the symposium is therefore obvious, since strategies for sustainable management, using precautionary reference levels of SSB and fishing mortality, are based on the relationship between SSB and recruitment. We need to move on to a longer term view which includes a wider range of information about the biological state of fish, their environment and their interaction with the rest of the marine ecosystem. Ed once more: “’Solving the recruitment problem’ is no longer the holy grail of fishery science. Appreciating recruitment variability, explaining probable causes, considering implications for management, and understanding it in the context of broader variability in marine ecosystems are worthy goals.”

Selected fishery and population parameters of eight shore-angling species in the Tsitsikamma National Park no-take marine reserve

An eight-year research angling dataset collected between February 1998 and December 2005 in the Tsitsikamma National Park marine protected area (MPA), along the south-eastern Cape coast of South Africa, was examined to provide estimates of important fishery and population parameters for eight important shore-angling species. Five different estimates of natural mortality (M), and the coefficients of variation (CVs) of catch per unit effort (CPUE) and size measurements were calculated. There was no consistency in estimates of M between methods, highlighting the difficulty in obtaining reliable estimates of this parameter for per-recruit assessments. The CVs of CPUE measurements were generally higher than those for size measurements. Estimates obtained for the most abundant species, blacktail Diplodus capensis, with a mean CPUE of 0.252 fish h–1, yielded the lowest CVs for CPUE (0.806) and size (0.130) measurements. Therefore, blacktail is considered the preferred indicator species for making comparisons with estimates obtained for shore-angling fish in exploited areas and other MPAs along the South African coastline. The large inter annual variations in mean CPUE and size for all species indicated that a minimum assessment period of four years is required to obtain reliable estimates.

A comparison of four types of sampling gear used to collect shovelnose sturgeon in the Lower Missouri River

Summary We compared variability in catch per unit effort (CPUE) and size structure of shovelnose sturgeon Scaphirhynchus platorynchus collected in 2003 and 2004 with stationary winter gill nets, drifted trammel nets, hoop nets, and otter trawls in the Lower Missouri River, USA to determine the most precise types of gear to collect all sizes of sturgeon so that refinements of longterm monitoring protocols can be made. A total of 1947 net sets or trawls collected 8743 shovelnose sturgeon, with 67% of the fish collected during winter gill netting (16% of total samples). Mean coefficient of variation (CV) among all months for juvenile (age 3 and younger; <250 mm fork length) sturgeon was highest for gill nets and lowest for otter trawls (P = 0.0008). Mean CV of subadult and adult shovelnose sturgeon (‡250 mm) was highest in hoop nets compared to other gear types (P = 0.0002). All gear and mesh sizes collected the most common sizes of shovelnose sturgeon (500–600 mm), but only otter trawls and trammel nets collected fish <150 mm. The higher precision of winter gill nets and summer otter trawls led to fewer samples needed to detect changes in CPUE as compared to hoop and trammel nets. Sampling only with types of gear that do not collect younger shovelnose sturgeon may hinder management decisions that rely on recruitment trends to determine the effects of management actions (e.g. channel modifications).

Diel variation in gillnet catches and vertical distribution of pelagic fishes in a stratified European reservoir

The diel changes in gillnet catch rates and vertical distributions of pelagic fishes were determined in a deep, moderately eutrophic European reservoir. In late summer, when steep vertical gradients in temperature and dissolved oxygen developed in the reservoir, monofilament gillnets (12.5–70 mm bar mesh) covering the upper 7.5 m of the water column were set at two open-water locations over 44- and 46-h periods, respectively. Gillnets divided the sampled strata (i.e., the epilimnion and metalimnion) into five horizontal layers each 1.5 m thick. Captured fish were removed from the nets at regular 2-h intervals. The catches were numerically dominated by bleak Alburnus alburnus, roach Rutilus rutilus, perch Perca fluviatilis, bream Abramis brama and asp Aspius aspius. Catch rates varied considerably with the time of day. The total gillnet catch per 2-h was on average 3–4 times higher at twilight than during the day or at night. Decreased net visibility and increased fish activity were likely responsible for the high gillnet catches at dusk and dawn periods. The catches of individual species were not distributed uniformly across the five sampled depth layers. The majority of bleak and asp were captured in the surface layer (0–1.5 m), irrespective of day, twilight or night. During the daytime, roach were most abundant in the layer just above the thermocline (3–4.5 m), but at night they occupied the upper part of the epilimnion (0–3 m). Perch inhabited the lower epilimnion and the upper metalimnion (1.5–6 m), and were caught only during daytime and twilight. For bleak and roach, greater proportions of larger individuals were caught during the day, while proportions of smaller fish increased in twilight and night catches. This implies that individuals of the two species made diel migrations between pelagic and littoral habitats.

MALTHUSIAN OVERFISHING AND EFFORTS TO OVERCOME IT ON KENYAN CORAL REEFS

This study examined trends along a gradient of fishing intensity in an artisanal coral reef fishery over a 10-year period along 75 km of Kenya's most populated coastline. As predicted by Malthusian scenarios, catch per unit effort (CPUE), mean trophic level, the functional diversity of fished taxa, and the diversity of gear declined, while total annual catch and catch variability increased along the fishing pressure gradient. The fishery was able to sustain high (approximately 16 Mg x km(-2) x yr(-1)) but variable yields at high fishing pressure due to the dominance of a few productive herbivorous fish species in the catch. The effect of two separate management strategies to overcome this Malthusian pattern was investigated: fisheries area closure and elimination of the dominant and most "competitive" gear. We found that sites within 5 km of the enforced closure showed significantly lower total catch and CPUE, but increased yield stability and trophic level of catch than predicted by regression models normalized for fishing effort. Sites that had excluded illegal beach seine use through active gear management exhibited increased total catch and CPUE. There was a strong interaction between closure and gear management, which indicates that, for closures to be effective at increasing catch, there must be simultaneous efforts at gear management around the periphery of the closures. We propose that Malthusian effects are responsible for the variation in gear and catch and that active management through reduced effort and reductions in the most competitive gear have the greatest potential to increase the functional and trophic diversity and per-person productivity.

Bottom-up control of common octopus Octopus vulgaris in the Galician upwelling system, northeast Atlantic Ocean

This paper investigates the possible underlying causes of the wide interannual fluctua- tions in catch of the common octopus Octopus vulgaris Cuvier, 1797 in one of the main small-scale fisheries off the coast of Galicia (northwest Spain). Galicia is at the northern boundary of the Iberian-Canary current upwelling system in the northeast Atlantic Ocean, where local winds induce seasonal upwelling, largely driving the annual cycles of primary and secondary production. We hypothesize that such dynamics are also fundamental for the survival of the planktonic stages of octopus and set the year class strength. We address this hypothesis by investigating the influence of upwelling on time-series of octopus fishery data. Wind stress structure during the spring-summer (prior to the hatching peak) and autumn-winter (during the planktonic stage) was found to affect the early life phase of this species, and explains up to 85% of the total variance of the year-to-year variability of the adult catch. Despite this bottom-up modulation via environmental conditions, our results also provide evidence for a between-cohort density-dependent interaction, probably caused by cannibalism and competition for habitat.

Beyond biological performance measures in management strategy evaluation: Bringing in economics and the effects of trawling on the benthos

The performance of management strategies for a prawn fishery in northern Australia is evaluated using the management strategy evaluation (MSE) approach. The operating model on which the analyses are based includes population dynamics models for four prawn species and five stocks of each species, an effort allocation model and a benthic impacts model. Management is implemented through controls on the fishing effort that targets the two main target species ( Penaeus semisulcatus and Penaeus esculentus) and the technical interactions between the two species are also taken into account. The total effort set by management is distributed to regions and grid cells in each region through effort allocation models. The performance measures used in this study cover conservation of the target species, economic returns and the impact of fishing on benthic communities. Two classes of management strategy are evaluated. One class seeks to move stocks towards the target spawning stock size which is a pre-specified fraction of the spawning stock size at which Maximum Sustainable Yield (MSY) is achieved using a threshold control rule, while the other class selects time-trajectories of future effort to maximize discounted profit. Management strategies that control effort levels to maximize the total profit over the long-term outperform those which aim to move the spawning stock size toward S MSY in terms of most performance measures. For example, even when the target stock size for the MSY-based management strategy is selected to be the same as that which maximizes profits, selecting effort to maximize profits leads to lower variability in catches and profits. This study illustrates how broader ecosystem considerations can be included in MSE analyses without the need for the development and implementation of full ecosystem models and hence provides a “middle road” between single-species MSEs and full ecosystem MSEs.

Practical stakeholder-driven harvest policies for groundfish fisheries in British Columbia, Canada

Fisheries co-management processes must provide a mechanism for industry stakeholder involvement in decision-making, while also providing assurance that precautionary actions will be taken to conserve fish stocks into the future. We used a management strategy evaluation approach to evaluate practical harvest policies suggested by industry stakeholders in a co-managed fishery for sablefish ( Anoplopoma fimbria) in British Columbia, Canada. These harvest policies included (i) data-based procedures that average recent catch limits with a smoothed research survey index of abundance and (ii) model-based procedures that rely on annual catch-at-age stock assessment modeling to estimate stock biomass. Both approaches attempt to implement constant exploitation rate harvest policies. We evaluated these procedures in four simulation scenarios that encompassed some important uncertainties related to current stock size and productivity. Both procedure types performed close to a perfect-information procedure in terms of catch, catch variability, and conservation, provided that exploitation rate policy parameters were set close to their optimal values. The smoothing function used in data-based procedures caused lags in which declines (increases) in catch limits extended for longer periods than declines (increases) in stock biomass. However, these lags did not create long-term adverse effects on performance. Model-based procedures generally performed better in terms of catch and inter-annual variability in catch. Interactions between harvest policy exploitation rates and stock assessment model biases caused similar lags as those of data-based procedures, although such biases also did not degrade performance severely. Our results, combined with empirical experience elsewhere suggest that data-based management procedures provide an appealing and practical means of setting annual catch limits either in the absence of an accepted model-based approach, or preferably, in combination with periodic stock assessment modeling. Such an approach provides transparency in a co-management process, while sacrificing little in terms of long-term conservation and utilization.

Harvest patterns and effort dynamics of indigenous and non-indigenous commercial sectors of the eastern Torres Strait reef line fishery

The reef line fishery (RLF) in eastern Torres Strait (ETS) is unique in that it has both a commercial indigenous sector and a commercial non-indigenous sector. Recently, concerns have been expressed by all stakeholders about the long-term sustainability of the fishery. These concerns have been exacerbated by the lack of detailed catch and effort information from both sectors, which has precluded any formal assessment of the fishery. In this paper, we characterise the harvest patterns and effort dynamics of the indigenous and non-indigenous commercial sectors of the ETS RLF using a range of data sources including commercial logbooks, community freezer records, voluntary logbooks and observer surveys. We demonstrate that bycatch is a significant component of the catch for both sectors and identify substantial differences in harvest patterns and effort dynamics between the sectors. Differences between sectors were observed in species composition and spatial and temporal patterns in catch, effort and catch per unit effort. These results highlight the inherent variation in catch and effort dynamics between the two commercial sectors of the ETS RLF and provide valuable information for the development of future assessments and appropriate management strategies for the fishery. The more reliable estimates of harvest patterns and effort dynamics for both sectors obtained from observer surveys will also assist in resolving issues relating to allocation of reef fish resources in Torres Strait.

Short- and long-range dispersal of the Queensland fruit fly, Bactrocera tryoni and its relevance to invasive potential, sterile insect technique and surveillance trapping

Dispersal of immature and sexually mature Queensland fruit fly, Bactrocera tryoni (Froggatt) from releases made at a single point was assessed from recapture rates obtained by using arrays of traps. The recapture data (pertaining to distances up to 480 m) fitted both logarithmic and Cauchy models although the fits for the releases of immature flies were inferior because of high variability in catches at certain distances. When combined with data previously published for longer distances, a Cauchy model fitted data for releases of immature flies well and indicated that the median distance dispersed after emerging from the puparium was ~120 m and that 90% of flies would displace less than 800 m despite the fact that a consistent trend in declining catch rates can be obtained up to at least 85 km. This is consistent with the tail of the Cauchy distribution having a slope congruent with a negative power curve and thus being scale invariant for longer distances. The distribution of recaptured flies that were released as adults also fitted a Cauchy model with a tail of the same slope, suggesting that the spatial distribution of long-distance dispersers is not only scale invariant but also age invariant. This has significance to the ability of surveillance trapping arrays to detect infestations and also to methods of distributing insects for the sterile insect technique. Whereas the spread of invading propagules in the first generation is likely to be limited by a decline to non-viable density within 1 km or less of the incursion point, the influence of larger infestations on nearby uninfested regions would be limited by the longevity of the dispersers.

Determining diel variation in fish assemblages downstream of three weirs in a regulated lowland river

The diel variation in boat electrofishing catches was investigated on the lower Murray River (south‐eastern Australia), downstream of three low‐level weirs. Fish assemblage composition did not differ among three sites, but differed substantially between day and night. Significantly more small‐bodied species (e.g. Australian smelt Retropinna semoni, flyspecked hardyhead Craterocephalus stercusmuscarum, bony herring Nematalosa erebi and Murray rainbowfish Melanotaenia fluviatilis) and some large‐bodied predators (e.g. golden perch Macquaria ambigua) were caught during daytime samples. Significantly more flatheaded gudgeon Phylipnodon grandiceps, carp gudgeon Hypseleotris spp. and perch Perca fluviatilis were collected at night. The size composition of the fish assemblage also changed over the diel cycle. Significantly smaller goldfish Carassius auratus, flyspecked hardyhead, bony herring and Australian smelt were sampled during the day, while generally smaller individuals of golden perch and the common carp Cyprinus carpio were caught at night. These findings suggest that sampling during both day and night is required to adequately characterize riverine fish assemblages in the context of fish migration studies.

Variability in Estimating Abundance of Postlarval Brown Shrimp, Farfantepenaeus aztecus (Ives), Migrating into Galveston Bay, Texas

Three sets of monitoring data were used to examine the variability associated with abundance estimation of postlarval brown shrimp, Farfantepenaeus aztecus (Ives) in Bolivar Roads, Texas—the main connection between the Gulf of Mexico and Galveston Bay. Abundance of postlarvae (PL) caught with Renfro beam trawl varied greatly in different years on the same dates. A “spring peak” of brown shrimp PL migrating into Galveston Bay was found for 2 April with a quadratic regression fit to 6-day moving averages of daily mean abundance from 22 yrs of monitoring data: Ln(PL+1) = 0.8736 + 0.09037Day 0.0004934Day2 (adj-R2 = 0.83, n = 159), where Day is Julian day. Abundance varied by four orders of magnitude (0 to 24,616 PL/tow) in just 4 d during a four-week intensive monitoring of PL during the 1987 spring peak. Abundance also varied by three orders of magnitude between the North and South Jetty sites during the same collection time. During a third study, PL abundance varied by two orders of magnitude along 360 m of the beach in < 4 hr. These investigations demonstrate that detecting significant differences in PL shrimp abundance in a pass requires substantial sampling that may not be logistically possible. However, best estimates could be obtained by including as many dates as possible, followed by including more sites, and finally by collecting during both day and night. Conclusions drawn from abundance studies of PL shrimp, fish, and crab immigrants through estuarine passes that are based on only a few samples should be reviewed. Geoffrey A. Matthews NOAA Fisheries, Galveston Laboratory, 4700 Ave U, Galveston, Texas 77551, e-mail: geoffrey.matthews@noaa.gov IntroductIon The brown shrimp, Farfantepenaeus aztecus (Ives), is a key commercial species in the shrimp fishery of the northwestern Gulf of Mexico (GOM). Most adults inhabit water depths of 20-65 m (Darnell et al. 1983, Neal et al. 1983) and spawning and larval development occur in these waters. Postlarvae (PL) migrate into the bay where they grow for about three months in salt marshes (Zimmerman and Minello 1984). Then, as advanced juveniles or sub-adults, they migrate back through the bays to the GOM, during which time they recruit to the bait and bay shrimp fisheries. All shrimp fisheries are valuable, are managed based on age-0 individuals (J. Nance and F. Patella, pers. comm., NMFS, Galveston, TX), and are characterized by large variability in annual catches (Klima et al. 1986). It is beneficial to commercial shrimp fishers and resource managers to have a forecast of the upcoming harvest, and the abundance of immigrating PL is a potential indicator of shrimp harvest (Baxter 1963, Berry and Baxter 1969, Baxter and Sullivan 1986). Various attempts to establish an early forecast using PL abundance have been unsuccessful (Williams and Deubler 1968, Berry and Baxter 1969, Sutter and Christmas 1982, DeLancey et al. 1994). These forecasting models have relied upon three important assumptions: (1) mortality rates for young brown shrimp in the estuary are either constant or vary in a regular manner seasonally from year to year; (2) the majority of recruitment of PL shrimp to estuarine nurseries occurs during the same months each year; and (3) accurate estimates of PL immigration to bays and estuaries have been obtained. Mortality rates of juvenile shrimp can be highly variable on a weekly or annual basis, but few measurements of this mortality are available (Minello et al. 1989). Accurate estimates of the influx of PL might not be possible; even the precision of such estimates has been studied only to a limited degree (Berry and Baxter 1969, Caillouet et al. 1968, 1970, Lochmann 1990). Only about 60% of the age-0 shrimp recruit to the fishery during the early summer, the rest recruit mostly during the next four months. The PL for the summer recruitment enter the estuaries in late winter and early spring, and Berry and Baxter (1969) hypothesized that the magnitude of the spring peak immigration might control fishery recruitment for that year. However, during winter and spring Arctic frontal passages, when the water is chilled and blown out of the estuaries by north winds (i.e. during a “blue norther”), the immigration of PL is delayed (Wenner et al. 1998, Blanton et al. 1999, Benfield and Downer 2001). These events weaken temporally-dependent models, increase the variability in the rate of PL immigration, and increase the variability in estimated density obtained by sampling. Brown shrimp larvae grow and develop as plankton in shelf waters of the GOM, and many factors lead to a patchy distribution as the PL migrate towards shore and immigrate through passes into bays. As meroplankton, their distribution is governed by seasonal circulation patterns, shelf gyres, wind-driven coastal and tidal currents (Temple and Fischer 1965, 1967, Temple and Martin 1979), and by wind and temperature controlled upwelling and downwelling (Wenner et

Evaluation of threshold management strategies for groundfish off the U.S. West Coast

Monte Carlo simulations and equilibrium analyses are used to explore the behavior of threshold management strategies. The analyses explore the medium- to long-term implications of uncertainty about the steepness of the stock–recruitment relationship, variation as well as possible temporal autocorrelation in recruitment, and estimation and implementation uncertainty. The results highlight the trade-offs among various management strategies, in particular the trade-offs between average catch, inter-annual variation in catch, and the risk of dropping below the Pacific Fishery Management Council (PFMC) overfished threshold of 25% of the average unfished spawning biomass. Uncertainty regarding steepness is the major source of variation in the final size of the resource and whether it is below the overfished threshold, although the extent of recruitment variability also impacts these quantities. The extent of inter-annual variation in catches is determined primarily by the amount of implementation error. Although the values for the various performance measures depend on all of the factors examined to some extent, the impact of interactions among these factors is generally small.

Influence of environmental factors on seasonal changes in clupeid catches in the Kigoma area of Lake Tanganyika

An investigation into the relationship between the fluctuating physico-chemical environment and variability in fish catches in the Kigoma, Tanzania, area of Lake Tanganyika was conducted from January to December 2003. Catch per unit effort (kg fishing unit–1 night–1) showed two strong peaks in February and August–September, which followed peak catches of Stolothrissa tanganicae. Stolothrissa was the most abundant species in the pelagic catches, followed by the centropomid Lates stappersi and Limnothrissa miodon. Chlorophyll a concentrations correlated positively with high clupeid catches. Wind speed accounted for over 56.7%, of the variability in clupeid catches, while chlorophyll and phosphate, accounted for 44.0% and 55.4%, respectively. The results confirm seasonal fluctuations in pelagic fish catches and the dependence of the lake's hydrodynamics on the weather system prevailing in the lake region. However, fluctuations in nutrient concentrations in the photic zone were less apparent than in previous studies. Regression analysis indicated that environmental factors have a significant impact on the fluctuations of the pelagic fish catches. More multidisciplinary data are required to confirm the dependence of clupeid catches on environmental factors.

Mortality and Exploitation Rate Estimates for the Recreational Bay Scallop Fishery off the Gulf Coast of Florida, USA

AbstractHistorically, the bay scallop Argopecten irradians supported recreational and commercial fisheries along Florida's Gulf Coast. Declines in density led to a ban on commercial fishing and tighter regulation of recreational fishing. In support of sustainable management, we derived estimates of mortality and exploitation from a multiyear series of preharvest and postharvest surveys of bay scallop density at five locations; catch curves from two locations during 3 years; and an aerial survey of recreational effort. Surveys at locations not subject to harvest yielded natural mortality estimates from which we derived estimates of fishing mortality and exploitation. Based solely on preharvest and postharvest surveys, we estimated the seasonal exploitation rate (u) as 0.0684. Based on all relevant surveys of bay scallop density, we calculated means and 95% confidence intervals for actual seasonal mortality (A = 0.6365 ± 0.06833), daily instantaneous total mortality (Z = 0.0153 ± 0.00293), daily instantaneous natural mortality (M = 0.0145 ± 0.00314), and daily instantaneous fishing mortality (F = 0.0013 ± 0.0055; 0.0000 replaced the negative lower confidence limit in further calculations). We used the means and confidence limits for M and F to calculate a mean and range for u (mean = 0.0575, range = 0.0000–0.2335). Effort on weekends and holidays was significantly greater than effort on weekdays. Data on effort and harvest generated u‐values of 0.1244 at Homosassa and 0.1667 at Steinhatchee. Overall, our results confirm that natural mortality is substantially higher than fishing mortality for bay scallops. Simulations indicated that current levels of harvest, current bag limits, and potential variations in catches are unlikely to extirpate bay scallops if initial densities are at least 25 scallops/600 m2 and if u stays within the calculated range. Our data did not indicate significant compensatory or depensatory mortality.

Runoff- and monsoon-driven variability of fish production in East China Seas

To understand the confounding fishing effect and physical influence on fish production, catch time series in the East China Seas were analyzed. Principal component analysis partitioned 18 catch time series into interannual trends and variations. While the trends were attributed to growth in fishing effort, variations in catches were related to precipitation and monsoon wind speed. Correlations of catch variations with the physical variables suggest that land-based runoff and monsoon circulation of the diluted coastal water masses are the physical forces dominating catch variability and the influences are largely through the associated nutrient supply on primary production. Runoff inputs nutrients to the coastal ecosystem, while monsoons drive their distribution. Offshore diffusion of the coastal water masses by the summer monsoon increases distribution and efficiency of nutrients and has a positive effect on fish production. Southerly transport of coastal currents alongshore by the winter monsoon confines nutrient distribution and induces nutrient loss from the northern waters. This process reduces overall and northern production, but increases production to the south. A long-term variation in catches was identified, which corresponds to a trend in the local winter monsoon as well as large-scale atmospheric changes. Prediction of the catch variation by the local wind speed suggests that large-scale atmospheric circulation determines the trend in the local winter monsoon, and the local winter monsoon that drives nutrient distribution should be directly responsible for the long-term variation of fish production in the East China Seas.

Effects of sampling errors on abundance estimates from virtual population analysis for walleye pollock in northern waters of Sea of Japan

Effects of sampling errors on abundance estimates from virtual population analysis (VPA) were quantified with the bootstrap method for stock of walleye pollock in the the Sea of Japan. In the bootstrap method, individual fish measurements were resampled. A total of 1000 bootstrap samples were produced for each year from 1991 to 2001. The coefficients of variation (CV) of catch at age in 2001 ranged 6.1–33.1%. The CV of an abundance estimate in 2001 ranged 9.0–35.7%. Abundance estimates of the oldest age and the latest year, which had larger CVs than the other estimates, were sensitive to sampling errors. Effects of sampling errors became smaller when the catch at age had been accumulated over a few years. Although VPA includes various types of errors, only the sampling errors have room for improvement in reality. Quantifying the effect of sampling error on VPA estimates is essential for sound and efficient stock management, and is emphasized in this study.