Trawl Gear Performance Trials

D. J. Sterling, J. E. Mellors, R. Watson, (1990). “Trawl Gear Performance Trials.,” Torres Strait prawn project: A review of research 1986-88 (Queensland Fisheries, Brisbane, Australia). Chapter 11.

An accurate estimate of abundance is essential for fisheries management. Abundance estimates are often limited, due to insufficient information regarding the catch efficiency of the sampling gear. A measure of sampling gear efficiency will enable a realistic estimation of abundance. Trawl sampling gear efficiency, is the proportion of animals retained in the net relative to the total number in the path of the trawl (Kjelsohn and Johnson 1978). Estimating the efficiency of the sampling gear often causes serious problems for the analysis of trawl survey data. Gear efficiency involves many complex variables that are often insufficiently understood to predict their cumulative effects. The efficiency of trawl sampling gear varies not only for each species but also for different size classes and with varying environmental conditions (Kjelsohn and Johnson 1978). This unmeasured variability in trawl sampling gear efficiency, results in an error in estimating population size. Measuring this error remains a major and largely unresolved challenge in fishery science. Selectivity is closely related to gear efficiency and includes components of net mesh selectivity and fish or prawn behaviour. Mesh selectivity is the variation in fishing mortality with age, which is the differential escape of certain sizes of fish or prawns after they enter the mouth of the net (Gulland 1983). Both efficiency and selectivity describe in some way the performance of gear. The distinction made is that efficiency is a measure of the gear’s catch relative to the total population present in the area swept by the net. Mesh selectivity is a measure of the gear’s catch relative to the number of prawns that entered the mouth of the net. Gear efficiency includes the effects of mesh selectivity and any other processes of selectivity that occur when fish or prawns are initially stimulated from the sea bed into the mouth of the trawl. ” Errors that occur in these calculations affect the estimates of absolute abundance and the shape of length-frequency distributions. Errors in length-frequency distributions are due to the size-specific nature of sampling trawl gear efficiency. Errors in estimates of absolute abundance are due both to the size-specific nature of the trawl, and to species-specific factors such as diurnal and seasonal variations in behaviour. Other factors that affect trawl efficiency are associated with the physical aspects of the fishing gear itself (Table 1 ). These factors have an importance that is often not fully appreciated in fisheries research. It is imperative that these factors are closely monitored during survey work to ensure that inter-sample variation of trawl efficiency is minimized. Commercial trawl gear designs compromise gear efficiency to achieve a practical and safe operation. The designs are selective for commercial species and sizes. They may be strongly biased against catching trash species and collecting debris even to the extent of having a negative effect on their commercial catch.If using commercial gear in fisheries research it may be necessary to be aware and correct for any bias that may exist. We conducted two separate experiments for efficiency and selectivity in order to investigate sample bias. Experiment 1 was designed to measure the size-specific trawl efficiency. Experiment 2 investigated mesh selectivity in greater detail. Experiment 1 is reported in full. The results presented for Experiment 2 incorporate data from only the first two monthly surveys and provides a preliminary examination of the selectivity performance of a standard port net relative to a small mesh starboard net.