TRENT and PRISTAS SELECTIVITY OF GILL NETS 



m i+l + m i 

 13.3 14.6 15.9 17.1 18.4 19.7 20.9 



SPANISH MACKEREL 



YELLOWFIN MENHADEN - 30.0 



25.0 



25.0 

 50.0 



6.7 7.3 7.9 8.6 9.2 9.8 10.5 

 STRECHED MESH (CM) 



FIGURE 2. — Regression of -2a/b on the sum of mesh sizes (m ( + 1 

 + mi ) and estimates of mean selection length by mesh size for 

 seven species of fishes. 



was the best available to estimate the mean 

 length-mesh size relation. 



Standard Deviation-Mesh Size Relation 



The third assumption of Holt's method is that 

 the standard deviations of length between mesh 

 sizes estimate a common standard deviation. 

 Standard deviations for the selectivity curves are 

 shown in Table 2 by species and mesh-size pair. 

 Standard deviations tended to: increase with an 

 increase in mesh size for gulf menhaden, sea 

 catfish, and Spanish mackerel; decrease with an 

 increase in mesh size for bluefish; and show no 

 apparent trend in relation to mesh size for Atlan- 



tic croaker, blue runner, and yellowfin menhaden. 

 Although only two estimates were available for 

 each species, standard deviations appeared simi- 

 lar between mesh-size pairs for spot and pigfish 

 and increased with an increase in mesh size for 

 pinfish. 



Standard deviations were much smaller for the 

 species that were usually wedged in the meshes 

 (gulf menhaden, spot, Atlantic croaker, blue 

 runner, pigfish, and yellowfin menhaden) than for 

 those species that were frequently entangled in 

 the meshes or caught at different girths along the 

 body (sea catfish, pinfish, bluefish, and Spanish 

 mackerel). 



SPECIES CAUGHT IN 

 LESSER ABUNDANCE 



Twelve other species were caught in sufficient 

 numbers to warrant general comments (Table 1, 

 Figure 1). Florida pompano, Trachinotus caro- 

 linus; spotted seatrout; inshore lizardfish, Syn- 

 odus foetens; ladyfish, Elops saurus; and sand 

 seatrout, Cynoscion arenarius, usually were 

 enmeshed in the webbing near their maximum 

 girth, although the latter four species sometimes 

 were entangled by their teeth; gulf flounder, Par- 

 alichthys albigutta, usually were enmeshed just 

 behind the opercle; crevalle jack and Atlantic 

 bumper, Chloroscombrus chrysurus, usually were 

 enmeshed but frequently were restricted by the 

 antrorse spine as described for blue runner; 

 gafftopsail catfish usually were enmeshed in the 

 larger mesh sizes but often were entangled by 

 pectoral and dorsal spines in the smaller mesh 

 sizes; little tunny and Atlantic sharpnose and 

 bonnethead sharks usually were entangled in the 

 webbing by their teeth and fins. In general, the 

 magnitude of the standard deviations reflects the 

 amount of entanglement. Standard deviations 

 were lowest for those species normally caught 

 wedged in the meshes and highest for those that 

 were frequently caught entangled (Table 1). 



Three of the above-mentioned species — spotted 

 seatrout, Florida pompano, and sand seatrout — 

 are important in the gill net fisheries along the 

 Gulf of Mexico. Although selectivity was not 

 evaluated for these species, owing to insufficient 

 data, estimates of the mean length-mesh size 

 relation can be made from the data in Figure 1. 

 The mean length plotted in Figure 1 would un- 

 biasedly estimate this relation only if equal 

 numbers of fish of each length class and species 



193 



