790 



Abstract— Although subsampHng is 

 a common method for describing the 

 composition of large and diverse trawl 

 catches, the accuracy of these tech- 

 niques is often unknown. We deter- 

 mined the sampling errors generated 

 from estimating the percentage of 

 the total number of species recorded 

 in catches, as well as the abundance 

 of each species, at each increase in 

 the proportion of the sorted catch. We 

 completely partitioned twenty prawn 

 trawl catches from tropical northern 

 Australia into subsamples of about 

 10 kg each. All subsamples were then 

 sorted, and species numbers recorded. 

 Catch weights ranged from 71 to 445 kg, 

 and the number offish species in trawls 

 ranged from 60 to 138, and invertebrate 

 species from 18 to 63. Almost 70% of the 

 species recorded in catches were "rare" 

 in subsamples (less than one individual 

 per 10 kg subsample or less than one in 

 every 389 individuals). 



A matrix was used to show the in- 

 crease in the total number of species 

 that were recorded in each catch as the 

 percentage of the sorted catch increased. 

 Simulation modelling showed that sort- 

 ing small subsamples (about 10% of 

 catch weights) identified about 50% of 

 the total number of species caught in a 

 trawl. Larger subsamples (509c of catch 

 weight on average) identified about 80% 

 of the total species caught in a trawl. 



The accuracy of estimating the abun- 

 dance of each species also increased 

 with increasing subsample size. For 

 the "rare" species, sampling error was 

 around 80% after sorting 10% of catch 

 weight and was just less than 50% after 

 40% of catch weight had been sorted. 

 For the "abundant" species (five or 

 more individuals per 10 kg subsample 

 or five or more in every 389 individu- 

 als), sampling error was around 25% 

 after sorting 10% of catch weight, but 

 was reduced to around 10% after 40% 

 of catch weight had been sorted. 



Does the size of subsamples taken from 

 multispecies trawl catches affect estimates 

 of catch composition and abundance? 



Donald S. Heales 



David T. Brewer 



CSIRO Marine Research 



233 Middle St 



Cleveland, Queensland 4163, Australia 



E mail address (for D S Heales) don.heales@marine csiro.au 



You-Gan Wang 



Dept of Biostatistics 



Harvard University 



Boston, Massachusetts 02115 



Peter N. Jones 



CSIRO Mathematical and Information Sciences 



233 Middle St., 



Cleveland, Queensland 4163, Australia 



Manuscript approved for publication 

 9 March 2003 by Scientific Editor. 



Manuscript received 26 June 2003 at 

 NMFS Scientific Publications Office. 



Fish Bull. 101:790-799 (2003). 



Concerns are held worldwide regarding 

 the sustainability of bycatch species 

 taken in trawls, particularly prawn 

 trawls. Under the voluntary FAO Code 

 for Responsible Fisheries, managers are 

 required to "take measures to conserve 

 target species, associated or dependent 

 species and nontarget species and their 

 environment" (FAO, 1995). An essen- 

 tial part of this process is the accurate 

 monitoring of population sizes and 

 structures. 



With large trawl catches, subsam- 

 pHng is often the only cost-effective 

 or feasible way to describe the bycatch 

 composition. How well these subsam- 

 ples represent the total catch depends 

 on how diverse the catch is, how well 

 the catch is mixed before the subsam- 

 ples are taken, and what proportion of 

 the catch is taken as a subsample. 



There is a large literature on sub- 

 sampling theory for terrestial insect 

 studies (Van Ark and Meiswinkel, 

 1992), aquatic macroinvcrtcbrate stud- 

 ies (Vinson, 1996; Walsh, 1997), and 

 marine ecological studies (Andrew and 

 Mapstone, 1987). However, in most of 

 these studies, samples of very small 

 animals collected in the field can be 

 resuspended in fluid and mixed evenly 

 in the laboratory before the subsam- 



ples are taken. In fisheries, in direct 

 contrast, large catches are extremely 

 difficult to manipulate and redistrib- 

 ute evenly before subsampling. A few 

 fisheries studies have examined the 

 impact of subsampling on estimates of 

 the abundance and different size ranges 

 of one or a few dominant species. For ex- 

 ample, in the Crangon trawl fisheries in 

 Belgian waters, sampling strategy had 

 only a minor effect on the reliability of 

 estimates of size selectivity for the tar- 

 geted shrimp (Polet and Redant, 1999). 

 In UK waters, subsampling trawled fish 

 (both target species and discards) from 

 either the sorting conveyor or the pound 

 made no difference to catch composition 

 estimates {Tamsett et al., 1999). 



However, in tropical trawl fisher- 

 ies, over one hundred species can be 

 recorded in a single catch. Under ESD 

 (ecological sustainable development) 

 guidelines, all these species (both tar- 

 get and bycatch) are equally important 

 but there has been very little research 

 on subsampling techniques applicable 

 to such diverse catches. A recent study 

 in Australia's Northern Prawn Fishery 

 (NPF) examined the accuracy of sub- 

 sampling from large, diverse catches 

 of fish and invertebrates (Heales et 

 al., 2000). For most of the "abundant" 



