Variation In Catch Research Articles

Space–time variation in catchability of southern rock lobster Jasus edwardsii in Tasmania explained by environmental, physiological and density-dependent processes

A seasonal catchability model for the rock lobster Jasus edwardsii, which had been developed previously for a population in a scientific reserve, was applied to catchability estimates over several years in northern and southern fishing regions off Tasmania, Australia. Catchability was estimated from commercial catch and effort data and fishery-independent estimates of exploitation rates. The seasonal catchability models describe the effects of water temperature, moulting and mating on catchability. They suggest that similar environmental and physiological processes underpin seasonal catchability in the two regions of the fishery, but that the relative importance of these factors varies considerably between the two regions. Physiological processes dominate the pattern of catchability in the north while water temperature contributes significantly to the model only in the south. Interannual variation in relative catchability was correlated with density-dependent processes. Full models described 72% of the total variation in catchability over 6 years in the south and 80% of the total variation over 4 years in the north.

MODIFICATION OF MARINE BENTHOS BY TRAWLING: TOWARD A GENERALIZATION FOR THE DEEP OCEAN?

Anthropogenic disturbance of deep-sea benthic systems, especially by fish- ing, has increased markedly in the last 40 years. Deep-sea mining and extraction of fossil fuels can occur at extraordinary intensity at individual sites, but the large number of fishing vessels and their mobility probably makes commercial trawling the most pervasive of our marine activities to depths of up to about 1200 m. Knowledge of the effects of trawling on soft-sediment, benthic communities is, however, limited to shallow, coastal systems, mostly at small spatial scales. We extend that knowledge to deeper systems at the scale of commercial fishing by assessing the effects of bottom trawling in northeastern New Zealand. We characterized the invertebrate catch of 66 research trawls spread along 220 km of continental slope in depths of 200-600 m (encompassing about 2400 km 2 ). At each site, we indexed the intensity of previous trawling using trawl-by-trawl fishing returns. A suite of multivariate analyses revealed that fishing activity was negatively associated (after ex- cluding the effects of depth and location) with invertebrate species richness and diversity and with the density of several taxa. Our models explained up to two thirds of the variation in the invertebrate catch of research trawls. After excluding the effects of depth and location, we attributed 11-40% of total variation to fishing. Concordance among the results of several multivariate methods based on different analytical approaches and assumptions reveals a strong and coherent pattern in the data that is consistent with the predicted and observed effects of trawl fisheries and other sources of physical disturbance. We infer that trawling probably changes benthic community structure and reduces biodiversity over broad spatial scales on the continental slope as well as in coastal systems. Such effects would have important implications for local and regional biodiversity and for the development and management of sustainable fisheries.

An Analysis of Methods for Quantifying Crappie Recruitment Variability

Abstract We used catch-at-age data from 122 populations of crappies Pomoxis spp. from across the midwestern and southeastern United States to compare two methods for defining recruitment variability: (1) the recruitment coefficient of determination (RCD), calculated from the weighted linear regression of loge(catch at age + 1) on age, and (2) the recruitment variability index (RVI). Ranked values of the RCD and RVI were significantly correlated, which suggests that both indices measured variability in catch-at-age data in a similar manner. The recruitment categories of RCD and RVI agreed 52% of the time or were within one category of each other 89% of the time. Modeling indicated that the RCD and RVI provided theoretically valid measures of recent recruitment variability but were not suitable for assessing long-term recruitment patterns. Ranked values of RCD and RVI were not significantly associated with ranked coefficients of variation (100·SD/mean) in catch per unit effort (catch per trap-net night) of ...

The effect of trawl speed on the footrope capture efficiency of a survey trawl

The effect of speed through water on the footrope capture efficiency of a survey bottom trawl was experimentally investigated by repetitively towing at three vessel speeds (2.5, 3.0 and 3.5 knots) with an auxiliary net attached underneath the trawl footrope to capture fish escaping beneath the trawl. Length-dependent capture efficiencies were then computed from the number of fish caught in both the trawl and auxiliary net. Capture efficiencies for Pacific cod Gadus macrocephalus, walleye pollock Theragra chalcogramma and Pacific halibut Hippoglossus stenolepis were not affected by fish length or trawl speed. Capture efficiency for skates Bathyraja spp. decreased with increasing trawl speed but was not affected by fish length. Capture efficiencies for arrowtooth flounder Atheresthes stomias and flathead sole Hippoglossoides elassodon increased with fish length and decreased with increasing trawl speed. Our results indicate that in areas of variable current, variation in survey catch per unit effort could be reduced for some species by standardizing towing speed to speed through water or by reducing footrope lift by the addition of sufficient weight to the trawl footrope.

THE ECONOMICS OF MARINE RESERVES

ABSTRACT. Marine reserves can be a useful supplement to other methods of fisheries management, but marine reserves alone are not likely to achieve a great deal in economic terms andperhaps not even in terms of conservation. The effects of marine reserves with open access elsewhere are analyzed, using a logistic model for a population with a patchy distribution. It is assumedthat a marine reserve is establishedfor the territory of one of two sub‐populations which interact through migrations. The total population increases while the total catch declines for the most part. A high rate of migration would, however, dilute the conservation effect.Examining a stochastic variant of the model shows that the variability (sum of squareddeviations) of catches may decrease as a result of protecting one of the sub‐populations. Even if all rents disappear by assumption, it is possible to identify this as an economic benefit, particularly when the average catch increases. The variability of the catch falls for a range of values of the population migration parameter and variability of growth, both when the stochastic disturbances are independent and when they are perfectly correlated for the two sub‐populations, andalso when the growth variability parameter differs between the sub‐populations.

The Contribution of Unmeasurable Inputs to Fisheries Production: An Analysis of Technical Efficiency of Fishing Vessels in the English Channel

AbstractVariations in the efficiency of demersal trawlers operating in the English Channel were examined through the estimation of a stochastic frontier production function. The most important measurable factor affecting efficiency was the age of the vessel. However, unmeasurable factors accounted for about 65% of the variation in efficiency, and as much as 9% of the total variation in catches between boats. It is postulated that most of this variation was due to differences in skipper and crew skill. In contrast, “luck” accounted for around 11% of the variation in catches between boats.

Variability of, and the influence of environmental factors on, the recruitment of postlarval and juvenile Penaeus merguiensis in the Matang mangroves of Malaysia

Postlarval and small juvenile Penaeus merguiensis were sampled fortnightly by beam trawls on the ebb tides over 4 years in the Matang mangroves on the west coast of peninsular Malaysia. Larger juveniles were sampled by bag nets on both the ebb and flood tides every fortnight over 5 years. Rainfall is consistently high throughout the year (>100 mm in each month) and the annual rainfall ranges from about 1,800 to 2,600 mm. Water temperatures ranged from 27°C to 36°C and salinity from 14‰ to 32‰ during this study. Fortnightly catches by beam trawls and bag nets were highly variable but showed a single broad period of postlarval and juvenile recruitment extending from December to May and an extended broad period of juvenile emigration from February to July. Beam trawl catches of postlarvae and juvenile prawns were consistently low between June and August. Higher catches of postlarvae and juveniles were recorded at two sites in close proximity to small adjoining creeks than one with no adjacent creek. Catches of juveniles in bag nets were higher during the day than the night and were correlated with the strength of the water flow. Environmental variables generally accounted for only a small proportion of the variation in catches, even for rainfall. The small proportion of variation explained by environmental variables is probably because of their relatively small seasonal variation in the Matang region. The lack of environmental influence on the catches of P. merguiensis implies that managing the mangrove nursery habitats and fishing pressure may be the main considerations for sustaining their stocks in the Matang region.

Rod catch trends for early-running MSW salmon in Scottish rivers (1952–1997): divergence among stock components

Temporal and spatial variations in rod catches of Atlantic salmon for the months February-June were examined for evidence of coherence using data for seven Scottish rivers, from 1952-1997. High levels of coherence were detected among rivers and among months. Examination of the combined catches for all Scottish rivers showed that rod catch trends over years differed among months. Marked declining trends over years were evident for February, March and April. The trends weakened progressively throughout the monthly series and rod catches were more stable in May and June. The divergence among months of trends over years is principally a feature of the later part of the time series (since ca. 1990). Marine and estuary net catches were combined with rod catches to estimate abundance prior to all the homewater fisheries. This suggests that abundance has declined for all the months examined. The contraction of marine and estuary fisheries has resulted in proportionately greater escapement to rivers, reducing the response of rod catches to underlying trends in pre-fishery abundance. We suggest that variation among months of rod catch trends over years is attributable to variations in marine mortality rate. Variation among monthly trends may reflect spatial variation in marine mortality rates among sub-catchment populations. Environmental effects acting on freshwater populations of juveniles in a spatially variable manner may subsequently cause parallel variation in marine mortality rates. Alternatively, variations in marine mortality rate may be attributable to population-specific migrations through spatially variable marine environments.

Characterizing catch variability in a multispecies fishery: implications for fishery management

Exploiting several fish species simultaneously reduces variability in daily catches. The reduction depends on the number of species, the catch-frequency distributions of individual species, and the level of co-occurrence of species in the catch. We explore theoretically the reduction of variability (coefficient of variation; CV) in the total catch by combining the distributions of daily catches of individual fish species, including zero catches, into a total catch frequency distribution. Theoretical findings are tested with an example from a stationary lift-net fishery for schooling small pelagic species around Ambon Island in the Central Moluccas, Indonesia. This fishery catches over 30 species, all with high daily variability (CV = 2.2–13.4). The reduction of variability in the total catch (CV = 1.7) is a result of the dominance and independent occurrence of the three main species. We conclude that in this fishery the information value of the total catch as an indicator of the catches of the individual species is low.

In search of a better unit of effort in the coastal liftnet fishery with lights for small pelagics in Indonesia

Despite major criticism, catch per unit effort (CPUE) is still widely used as a measure for the size of the exploited stock, but its indicative value is affected by selection of a proper unit of effort. The unit of effort used in the Indonesian fisheries statistical system is poorly standardised with one trip made by a fishing unit of any type. Therefore, changes in catchability, induced by technical innovations cannot be accounted for in official statistics and thus bias the perception of fisheries authorities on the state of stocks. This study shows that differences in technical characteristics in a coastal liftnet fishery on small pelagics around Ambon, such as unit size and type of lamps used to attract fish, influence both effort allocation and catch per trip. Larger fishing units (more windlasses) are more commercially orientated. They have a larger nonzero catch per trip—1.5 times larger nonzero catch per added windlass—and ignore potentially small catches, as shows from their higher proportion of zero-catches. The shift from kerosene to electric lamps does not lead to a higher catch per trip, but the use of electric lamps is less laborious, inviting fishermen to fish also under more unfavourable light conditions such as during full moon. Model simulations of average catch per trip of two fishing areas show that technical innovations in the last 20 years have increased the mean catch per trip by a factor of 4.1 in the fishing area with the largest technical improvements. Technical improvements explain 80% of the change in catch per trip in the official statistics over the last 15 years. Furthermore, the variance in catch per trip among individual fishing units has increased and so has the uncertainty in the average CPUE, as only part of the fishermen implemented technical improvements. Both developments stress the poor indicator value of catch per trip for developments in fish stock biomass and the importance to standardise the unit of effort. The characteristic high variability in catch in this fishery will always obscure effects of technical improvements in more local situations and smaller timeframes. Because of the inconsistent effects of the type of lamps used on the catch rate, it is questionable whether standardising the unit of effort by the type of lamp used will enlarge the precision in the CPUE. Nevertheless, standardising the unit of effort by the size of the unit will enlarge the precision in, and the indicator value of the CPUE in more aggregated catch data as used on higher levels in the fisheries administration of Indonesia.

Catchability of the southern rock lobster Jasus edwardsii. I. Effects of sex, season and catch history

Seasonal variation in catchability of the southern rock lobster Jasus edwardsii, was estimated in a scientific reserve in south-east Tasmania by comparing estimates of lobster density based on direct visual observations underwater with concomitant estimates from trapping surveys. Underwater density estimates of undersized and legal-sized male and female lobsters >80 mm carapace length, did not change significantly over the 14-month study period, with the exception of undersized males (≤110 mm carapace length). Sex ratios remained constant at approximately 1 : 1. In marked contrast, catch rates of males and females and the sex ratio of trapped lobsters varied strongly with season, implying that catchability varies seasonally and with sex. Catchability of males and females was highest in early summer and lowest in winter. Impact of capture on subsequent catchability appeared to be weak, since the ratios of tagged animals in the population observed underwater generally reflected recapture rates of tagged animals in trap catches. Recapture rates increased with size and were higher for medium-sized and large males than for similar-sized females. However, for each particular sex-size group, recapture rates remained relatively constant throughout the study period.

Diurnal variation in bottom trawl survey catches: does it pay to adjust?

The diurnal bias of bottom trawl survey catches is studied with the purpose of adjusting for it and thereby improving the accuracy of abundance estimates. The correction term is estimated with uncertainty and thus increases the variance of the resulting abundance estimate. To investigate this adequately, we use a stochastic model describing diurnal fluctuations and examine the annual variation of the diurnal amplitude as a function of species and length. The diurnal amplitude is fairly stable for large fish, and for these, the bias-corrected estimate leads to a moderate increase in variance. For small fish, the diurnal amplitude is unstable, however, and the correction of diurnal bias occurs at the expense of a large increase in variance. This unstable amplitude also leads to a large year-to-year variation in catchability for small fish. For haddock (Melanogrammus aeglefinus), the diurnal amplitude depends heavily on fish length, indicating a strong decrease in catchability with decreasing fish length.

Trophic state, fish community and intensive production of salmonids in Alicura Reservoir (Patagonia, Argentina)

AbstractThe Governments of the Provinces located in Patagonia, Argentina, promote the intensive breeding of salmonids in the Andean Patagonian region. Although annual production is low (450 ton ha–1 year–1), some effects are significant. Waste produced by salmonid breeding (feed losses, faeces and excretion) increases nutrient and organic matter concentrations, which cause modifications of water quality, sediments and biota. A consequent risk is the elevation of eutrophication levels. Possible changes in water composition, sediments, algae and wild fish populations were studied. Sites affected by fish farming showed increased nutrient concentration, and phytoplankton and periphyton biomass. Chlorophyll a was similar at both sites (affected and unaffected by fish farm sites). Sediments clearly reflect fish farm waste inputs: total phosphorus and organic matter increased 12‐fold and fourfold, respectively. The species present in the gill‐net catches were the autochthonous Percichthys trucha, Odontesthes hatcheri, Diplomystes viedmensis, and the introduced salmonids Oncorhynchus mykiss, Salmo trutta, Salmo salar sebago and Salvelinus fontinalis. About 50% of the total catch was salmonids. A major portion of the catch per unit weight was composed of rainbow trout, followed by perch. The catch per unit weight obtained for this reservoir agrees with the range of values previously determined (Quiros 1990) for Patagonian reservoirs. Compared with previous studies by Freyre et al. (1991), a variation in catch composition exists. This consists mainly of an increase in the numbers and condition of O. mykiss and a decrease in P. trucha. Presence of fish that escaped from hatcheries, recognizable by their eroded fins, was observed; particularly in a sampling station near the fish cage systems. Variations in catches could be caused by cyclical changes in fish populations (Wooton 1991), by direct and indirect effects of intensive fish farming, or by a combination of both events, and can only be understood through long‐term studies of catch variation.

Effect of Silhouette Color on Trap Catches of <I>Dendroctonus frontalis</I> (Coleoptera: Scolytidae)

With the exception of responses to semiochemicals, host selection behaviors of D. frontalis are largely unstudied. To better understand the host finding behavior of D. frontalis, and to identify potential visual disruptants, we evaluated the response of D. frontalis to multiple-funnel traps of eight different colors. Multiple-funnel traps provide an attractive vertical silhouette, similar to a host stem, that aids in capturing bark beetles and allows for controlled evaluation of visual cues. Evaluation of mean trap catch of each color by analysis of variance (ANOVA) produced two separate groups: white and yellow traps caught significantly fewer D. frontalis than the other six colors tested (black, blue, brown, gray, green, red). Examination of spectral reflectance curves showed that the eight colors could be naturally placed into two groups, those with high peak reflectance (white and yellow) and those with low peak reflectance (black, blue, brown, gray, green, red). When high and low peak reflectance were substituted for color in a separate ANOVA, reflectance group was as good as color at explaining the variability in trap catch (r2 = 0.88 versus 0.92). Therefore, hue (dominant wavelength) was unimportant in affecting D. frontalis host finding behavior at the reflectance levels we tested and, thus, we found no strong evidence that differential wavelength sensitivity affected the response of D. frontalis. These results show that dark colored silhouettes (those with low reflectance values), regardless of hue, are best for capturing D. frontalis, while white or yellow are the best candidate colors for disrupting host finding.

Risk aversion in allocating fishing effort in a highly uncertain coastal fishery for pelagic fish, Moluccas, Indonesia

The Ambonese small-scale purse-seine fishery for small pelagic fish, such as scads and mackerels, is characterised by highly variable daily catches. Fishermen involved in this fishery are therefore seriously constrained in optimising the outcome of their fishery through spatial allocation of effort. Spatial patterns in effort allocation were compared with those in catch per unit effort (CPUE), indexed by both catch weight and profit. Average CPUE indexed by catch weight differed between fishing locations by up to 14 times. However, individual fishermen could only detect such large differences after 14 days of exploratory fishing because of the high variability in daily catches. Daily decisions on effort allocation are therefore not based on maximising CPUE but on minimising operational costs and risk. A very high proportion (88%) of the fishing trips were made within 8 km of the home port, although the capacity of the purse seiners allowed for fishing in more productive areas much farther away. A 10- to 20-fold increase in operation costs (travelling and local use rights) when fishing in other areas reinforced this behaviour.

Constant catch or constant harvest rate?: The Baltic Sea cod ( Gadus morhua L.) fishery as a modelling example

Overfishing is a major problem in the world fisheries. Constant harvest rate strategies, with catches proportional to the abundance of the target species, stimulates investments when stock sizes are large, often resulting in an overcapacity when the stocks decrease. These investment incentives are weaker under a constant catch strategy, since there are less reasons to have a catching and fish processing capacity that is larger than the allowed constant catch. We have explored the potential of a management strategy based on constant catches, by modelling the fishery on the eastern Baltic Sea cod ( Gadus morhua L.). To avoid the low long-term yield (LTY) often resulting from a strict constant catch approach, we developed a quasi-constant catch (QCC) strategy based on alternative constant catch levels and two overfishing criteria. Normal catches are allowed when the spawning stock biomass is above a certain level. When the spawning stock biomass falls below this level, catches are reduced to a given extent. If the spawning stock biomass falls below a second and lower threshold, the fishery is closed. The QCC strategy was tested with a model based on the same basic premises as the multi-species virtual population analyses (MSVPA) for Baltic Sea cod, developed within ICES. In addition, recruitment processes were included and probability analyses were done to estimate different risks of overfishing. According to the model, the LTY with the QCC strategy was 10% lower than with the constant harvest rate strategy. However, the QCC approach had the advantage of smaller variation in catches, potentially resulting in a better utilisation of the fishery capacity. The capacity utilisation was better with the QCC strategy relative to the constant harvest rate strategy, especially in fisheries where catch quotas are based on assessments with substantial errors. Furthermore, QCC gave a better capacity utilisation in fisheries where the size of the fleet and processing capacity is non-flexible and when effective fish finding techniques are used. A more effective capacity utilisation could counterbalance the disadvantage of the lower LTY compared to the constant harvest rate strategy. The QCC approach is hence an interesting and competitive management strategy, that is likely to be more effective in avoiding the emergence overcapitalisation and overfishing of long-lived species like the Baltic cod. QCC may also allow catch quotas to be set for more than 1 year, provided that the stock size of the target species is relatively large.

REGULATION OF A FISHERY: FROM A LOCAL OPTIMAL CONTROL PROBLEM TO AN “INVARIANT DOMAIN‘ APPROACH

ABSTRACT. This study aims at regulating a fishery by reducing its catch and effort variations around a reference equilibrium point. A simple bioeconomic continuous time model is considered and the control variable acts on the rate of change of effort. Two approaches have been implemented: the first one is a local optimal approach that sets the problem in the classical linear‐quadratic framework; it is extended to an “invariant domain‘ approach, whose robustness constitutes a major advantage.

Can fishermen allocate their fishing effort in space and time on the basis of their catch rates? An example from Spermonde Archipelago, SW Sulawesi, Indonesia

Spatial and temporal patterns in catch rates and in allocation of fishing effort were analysed for the coastal fishery in Spermonde Archipelago, Indonesia, to assess whether fishermen can optimise their strategy from catch information, or whether they fish under great uncertainty and merely minimise risks. On average 517 fishing units operated in the 2800 km2area, catching 21 t fish day−1. Major gear categories were hook and line (59% of total effort and 5% of total catch), and lift nets (16% of total effort and 70% of total catch). The size of individual resource spaces varied with gear type and was smaller in unfavourable weather conditions. Although spatial patterns in catch rates at the scale of the whole archipelago were evident, fishermen could not differentiate between locations, as catch variance within their individual resource spaces was high relative to the contrasts in spatial patterns. The aggregated distribution of fishing effort in Spermonde must be explained by factors such as the small scale of operations, rather than fish abundance.

Relationship between freshwater angling catch of Atlantic salmon and stock size in the River Bush, Northern Ireland

Over 25 years rod catches of Atlantic salmon Salmo salar increased proportionately as stock size increased (r2=0·581, P<0·001), with no overall trend between exploitation rate and stock size (r2=0·016, P>0·5). On a 15 year sub‐set of these data annual effort (P=0·804) and flow (P=0·339) had little significance relative to stock size (P<0·01) on variation in rod catches. Stock size, time, effort and flows had no influence on inter‐annual variation of rod exploitation rate (r2=0·094, P=0·880). Pairwise correlation between variables confirmed these results. In 1998, weekly effort contributed significantly to overall catch variation (P<0·001), while weekly flow did not (P=0·438). These results are discussed in relation to the utility of rod catch data for deriving estimates of stock for spawning target compliance purposes.

Variation in tuna fish catches in pacific prehistory

AbstractThe archaeological evidence from Pacific faunal assemblages held in the Museum of New Zealand, Te Papa Tongarewa, is reviewed. An emphasis on inshore fish and fishing is demonstrated, while evidence for offshore fish, such as tuna, is slight. Yet ethnographic accounts of fishing in the Pacific often contain detailed descriptions of offshore fishing expeditions to catch pelagic fish, including tuna, and these fish continue to occupy a significant place in the cultural life of many Pacific island communities. The uneven representation of tuna in archaeological sites is not thought to derive from any known taphonomic process, but fairly represents the relative abundance of tuna in catches throughout the Pacific. Discontinuities in catch frequency do not follow any known variations in natural abundance, and the preferred explanation is cultural choice. Moreover, in sites where tuna were a major component of the catch, there appears to be a small but consistent decline in their relative abundance, again believed to reflect cultural–historical, rather than natural, processes. It is suggested that the social importance, as well as economic aspects of tuna fishing, need to be incorporated in explanations for the presence of tuna in archaeological sites. Copyright © 2001 John Wiley & Sons, Ltd.