Trap Haul Research Articles

Mesh size selection and diel variability in catch of fish traps on the central Great Barrier Reef, Australia: a preliminary investigation

The effect of mesh size and diel variability in fish trap catch was investigated on the central Great Barrier Reef in order to select an optimal mesh size for ecological studies of lutjanids (snappers) and lethrinids (emperors). Four mesh sizes, 12.5 mm square, 30 mm hexagonal, 40 mm hexagonal and 50 mm hexagonal were examined over two sampling periods (day versus night) yielding a total of 1571 fish representing 70 species from 451 trap hauls. Mesh size had no significant effect on the total catch rate of fish traps. The 40 mm hexagonal mesh size was, however, the preferred mesh size because the size range of target species ( Lutjanus and Lethrinus) caught in this mesh was greater than that in the 50 mm hexagonal mesh and the mean catch rate tended to be greater than the 30 mm hexagonal mesh. The mean catch rate in the 12.5 mm square mesh was low relative to the mean catch rates of the other mesh sizes tested and the size range of target species caught in the mesh size were not significantly different from that caught in the 30 mm hexagonal mesh. Diel sampling period had a significant effect on the catch of fish traps. More fish were captured at night, with the lutjanids (78% of total catch) and lethrinids dominating night and day catches, respectively. Mesh selectivity in fish traps was probably not a simple function of the ability of a given mesh aperture to retain individuals of a certain body depth. The behaviour and activity patterns of reef fish in response to the visual outline of the traps was considered of prime importance in the mesh selectivity.

Critique of the Leslie Method for Estimating Sizes of Crab and Lobster Populations

The Leslie method can be used to estimate the size of a decapod population by regressing catch per trap haul on cumulative catch of the fleet at several time intervals within a fishing season. The data requirements are modest and the calculations simple; however, demands for a time series of catch–effort data and for the capture of a large portion of the population entail assumptions that are easily violated. These assumptions are that the target species has constant catchability over the sampling period, the fishing effort is distributed uniformly over the fishing ground, the fishing methods do not change, the target population is closed, and landings and effort arc reported correctly. The assumptions were investigated by reviewing the literature for examples of deviations, and by simulating the deviations in a simple model. Petersen tag–recapture studies have been used to verify the accuracy of Leslie estimates. However, important assumptions are common to both methods, and violations bias stock size estimates in the same direction. Also, the cause of bias cannot be diagnosed from shape of the Leslie regression. Without knowing the cause, a correction cannot be made, However, some biases can be corrected with supplementary data if their cause is identified. Mean catch per unit effort used as an index of stock abundance is an alternative to the Leslie method. Simulations showed this method to be equally or more accurate than the Leslie method for several scenarios of decapod catchability and recruitment.

Studies of fish traps as stock assessment devices on a shallow reef in south-western Puerto Rico

Traditional Antillean arrowhead fish traps were deployed around and on a shallow, 8-ha coral reef of south-western Puerto Rico in two 1987 studies: Study I, 2–18 March, a comparison of 30 trap catches and 30 visual transect counts; Study II, 8 April–18 June, a tag and recapture study of 95 trap hauls. Study I considered model taxa and conditions for calculating effective fishing area (EFA) of the traps on the basis of catch per haul and fish density from transects. Of the 185 fish of 24 species caught, EFA could only reasonably be calculated for three taxa: redband parrotfish, Sparisoma aurofrenatum, 25 m 2 and 90 m 2; stoplight parrotfish, Sparisoma viride, 24 m 2; ocean surgeon, Acanthurus bahianus, 93 m 2. Other taxa were considered unsuitable because of near-zero density or catche, e.g. white grunt, Haemulon plumieri, was often caught but seldom seen. Ability to assess EFA depended on trap location; future surveys of reef fishes will necessarily have to utilize stratified sampling based on habitat characteristics. Study II considered fish movements within a five-trap, cross-shaped array deployed on a back reef area. Of 702 captures from 28 species, redband parrotfish, white grunt and ocean surgeon dominated. Of 272 fish tagged, 65 were recaptured once and 24 were recaptured twice or more. In the short term, traps seemed to sample a discrete population. Except for one individual, all tagged fish were recaptured at the same trap or at a nearby trap of the array, i.e. no more than 25 or 35 m away; ocean surgeon were always caught at the trap where tagged. Using the mark-recapture data, population estimates were calculated for dominant species. These were not used to estimate EFA because the area sampled by the traps could not be reliably estimated. However, properly designed marking studies could be used to obtain this information. It was concluded that using EFA in trap-based assessments is a promising technique for residents like redband parrotfish or ocean surgeon. Mark and recapture techniques may also return useful density estimates for cryptic or diurnally migrating residents such as white grunt.

Aggregations of Berried Lobsters (Homatus americanus) in Shallow Waters off Grand Manan, Eastern Canada

The relative abundance, size, and sex frequencies of trap-caught lobsters were determined from five areas near the northern end of Grand Manan during August–September 1982 and August 1983. Large numbers of berried (ovigerous) females (90/100 trap hauls) and few male or nonberried female lobsters were caught in shallow (1–22 m) depths close to the shoreline in one area. The sex ratio was near 50% males in deeper areas. In the shallow area SCUBA divers observed mature lobsters excavating bowl-shaped depressions in the sand-clay substrate and confirmed the high density of berried females indicated by the trap catches. Berried females may seek and congregate in areas favourable for egg development and larval dispersal. During summer, shallow areas provide warmer water than deeper areas and therefore allow faster egg development and egg extrusion. Also, the shallow waters with strong tidal currents would allow rapid dispersal of newly hatched larvae.

Population and catch structure of the edible crab (Cancer pagurus) in the English Channel

Cancer pagurus is widely distributed in the English Channel, supporting a fishery worth over £ 1 million along the south and southwest coasts of England. The most important area is off Devon where fishing extends up to 60 km offshore. Landings and catch rates vary seasonally, with the main fishery depending upon large mature female crabs with developing gonads, caught in the autumn on the offshore grounds at catch rates of up to 265 kg/100 trap hauls. Large male crabs, up to 267 mm carapace width and 4200 g in weight are caught, particularly inshore, during the spring. Larger crabs are found in deeper water and at the western end of the Channel. In the autumn the proportion of females in the catches increases, as does their mean size. Mean sizes have varied little in the period 1967 to 1974. Selection of the landed catch by fishermen is dependent upon market preferences, with average minimum selection size of about 150 mm; the legal minimum size is 115 mm carapace width. Carapace width-total weight relationship regression equations indicate allometric growth for males and isometric for females. Unlike stocks in the North Sea, where a well-defined moulting period exists, soft-shelled (recently moulted) crabs occurred throughout most of the year, though generally more were found in the summer. Development of the ovaries takes place from August to December, with up to 90% of the females, in November, having well-developed ovaries. Generally egg-bearing females were absent from the catches; a few were observed, mainly in the spring, with sizes ranging from 133 mm to 205 mm. Smaller females were found with plugs in the oviducts, indicating that mating had taken place, but they were not necessarily mature. The hepatopancreas/gonad meat yield of females varied seasonally, being related to moult condition and gonad development. Comparison between the population and catch structure of the English Channel and North Sea crab stocks shows considerable differences, with much higher catch rates and higher mean sizes in the Channel. Extensive migratory movements of mature female C. pagurus down the English Channel result in a higher proportion of females in the catches, compared with the eastern end of the Channel, and a higher mean size. This concentration in the autumn of female crabs with well developed gonads on the grounds off south Devon, is probably related to the breeding cycle, with the requirement for a suitable bottom substrate for spawning, and the distribution of the larval stages by residual currents.

Estimation of Catchabilities and Population Sizes of Lobsters

Techniques of estimating population size, level of fishing, and the degree of dependence of fishing success on environmental factors are examined on the basis of tagging, catch and effort data. A new method is developed to estimate population size from catch, effort, and temperature data when the catchability varies with temperature.The methods of estimation discussed are applied to data collected from a number of lobster fisheries on Canada's Atlantic coast. Analysis confirms a relationship between the catchability of lobsters and bottom temperature. Differences in this relationship are found between areas and between tagged and untagged lobsters within areas. It is suggested that these differences are attributable to the differences in densities as well as to aggregations of lobsters and fishing. The effect of these aggregations on population size estimates is considered.Calculated average catchabilities at comparable temperatures are different for different areas. These differences are correlated with the numbers of trap hauls per day per square miles fished. It is suggested that the differences in the catchabilities might be due to interactions between units of gear not predicted by the customary relationship between catch and effort.

The Activity and Catchability of the Lobster (Homarus americanus) in Relation to Temperature

The activity (walking rate) of lobsters acclimated to various temperatures increased from 2 to 10 °C. and from 20 to 25 °C. but was constant between 10 and 20 °C. Lobsters acclimated to low temperatures were most active above the acclimation temperature. Those acclimated to intermediate and high temperatures were most active at the acclimation temperatures. Experimental fishing of a relatively constant stock showed that the catch per 100 standard trap hauls (index of catchability) was linearly related to temperature. Slope of line is, however, dependent on stock density. Catchability is shown to be proportional to activity minus a constant. This relationship permits use of catch per unit effort and accumulated catch data to estimate lobster populations.

A new maturity test of peas

In most crustaceans, including the American lobster (Homarus americanus), a high frequency of individuals with missing claws is often reported. As lobsters with missing claws (“culls”) are less valuable in terms of weight and market value than uninjured lobsters, returning of culls to the sea has been suggested as a conservation strategy. To assess the potential benefits of such strategy, the frequency and types of claw loss were documented during the fishery season of 2007 (34 days, 6900 trap hauls) in an area of Prince Edward Island, southern Gulf of St. Lawrence. On average, culls accounted for ∼7% of the total catch in weight and such percentage remained fairly constant along the study period. The raw frequency of culls, however, was higher in the first 9–12 days of the study and decreased progressively afterwards, reflecting the pattern followed by actual total catches. A single missing claw (pincher or crusher in similar proportions) was the most common type of injury for both males and females, followed by the presence of a single regenerating claw. The male:female ratio for the culls (∼1:2) did not deviate from the one estimated for uninjured lobsters of the same size range (canners). These results are used to further discuss life history implications and the potential benefits and limitations of a conservation strategy based on the return of culls to the water.