Comparison of the Catch from Tongue and Two-Seam Shrimp Nets off South Carolina

Abstract

Before the gears were used for catch comparisons, a two-seam net and a tongue trawl were evaluated for changes in net dimensions with fishing depth and tow direction. When towed as it would be during catch comparisons, the two-seam net had a width of 16.1 m and was estimated to extend 2.1 m vertically at the center of the head rope. The hori/ontal spread of the tongue trawl was 13.5 m and its vertical spread was 4.2 m at center. Small, statistically consistent differences in openings (<0.5 m) occurred with depth and direction. The major factor influencing changes in catch (kg/ha) with depth (10-fold increase in shallow water) appeared to be the fauna! distribution with depth, independent of towing characteristics. Differences in biomass (kg/tow, kg/ha), and in the biomass (kg/ha) ratios of taxa to shrimp between the two-seam and tongue trawls were documented for eight major biological groups. Major differences in total catch by net occurred between years primarily because of changes in the catch of miscellaneous invertebrates and shrimp. Significant differences in the lengths of nine priority species occurred between the two gears. Mean lengths in the two nets differed by more than 1 cm for spot Leiostotnus xanthurus (which was larger in the tongue net), Atlantic croaker Micropogonias undulatus (larger in the two-seam net), and Spanish mackerel Scomberomorus maculatus (larger in the tongue net). Mean ratio of fish to shrimp biomass was 31:1 overall (21:1 for the two-seam net and 41:1 for the tongue trawl). Ratios of total biomass and the biomass of any taxonomic grouping to shrimp biomass did not differ statistically between the two gears. Biomass ratios were recalculated from published data by a standard methodology. Subsequent comparisons indicated increases in the ratios over time and highlighted a need to validate the technique of subsampling heterogenous trawl samples. Finfish by-catch in both gears was dominated by sciacnids (44% by weight of all fish). Red drum Sciaenops ocellatus, spotted scatrout Cynoscion nebulas us, snappers (Luljanidac), and groupers (Epinephelinae) were not caught by cither net. Catches of Spanish mackerel and king mackerel Scomberomorus cavalla were documented and warrant further investigation to evaluate the effects of by-catch on local populations.

Sampling estuarine fish and invertebrates using demersal otter trawls: Effects of net height, tow duration and diel period

“…These differences in catch rates were probably related to the greater volume of water above the substratum sampled by the high-rise net. Similarly, Stender and Barans (1994) reported larger catches of white shrimp, Penaeus setiferus, in a tongue net that also sampled a greater proportion of the water column.…”

“…Although otter trawls are easily utilised and relatively efficient at sampling large numbers and diversities of organisms, the catchability of these gears may be affected by a range of technical and operational factors. Some of these factors include: (i) the design of the net (DeAlteris et al, 1989;Merrett et al, 1991;Stender and Barans, 1994;Wantiez, 1996;West, 2002); (ii) the duration of tows (Godo et al, 1990;Walsh, 1991;Somerton et al, 2002); and (iii) diel period (Engas and Soldal, 1992;Korsbrekke and Nakken, 1999;Petrakis et al, 2001).…”

Modifying penaeid-trawl spreading mechanisms to improve environmental efficiency

“…It is clear, however, that there is delicate balance between optimising headline height for the targeted penaeids, while minimising teleost catches (Eayrs 2002;Stender and Barnes 1994; Madhu et al. 2015).…”

“…It is clear, however, that there is delicate balance between optimising headline height for the targeted penaeids, while minimising teleost catches (Eayrs 2002;Stender and Barnes 1994;Madhu et al 2015). In one of the first relevant studies, Hines et al (1999) compared high (3.7 m) and low (0.9 m) headline heights in skimmer trawls (same SR) and while the latter caught less total bycatch (by 365 ~14%), catches of brown shrimp were significantly reduced (by up to 39%).…”

“…McHugh et al In Review trawl, whereby a 'tongue' section in the centre of an extended bosom section divides the wide bosom into two smaller sections giving increased ground coverage (Thomsen et al 2004). While double 500 bosom trawls are not widely used to target penaeids (Stender and Barnes 1994), a similar conceptthe 'W' trawl-was developed by Balash et al (2015a). The W trawl extends the double bosom concept to the headline, and while its primary objective is to lower drag, there have been encouraging results for reducing bycatch (by up to 11%; Balash et al 2015a).…”