FISHERY BULLETIN: VOL. 75, NO. 1 



were available in the fished population — an 

 assumption that is not valid. Based on the low 

 standard deviations in length for each mesh size 

 (Table 1), however, it appears that a particular 

 mesh size would efficiently capture any of these 

 three species only over narrow length ranges. 

 When this situation exists, only a small amount of 

 bias in the mean length-mesh size relation results 

 from using the estimates derived by plotting the 

 empirical data. 



DISCUSSION 



Information presented in this paper can be used 

 in fisheries management and research, and by 

 commercial fishermen, in the following ways. We 

 categorized the uses into two types: mesh-size 

 regulations and capture efficiency. 



Mesh-Size Regulations 



Mesh-size regulations in a fishery should serve 

 specific purposes. These regulations can be useful 

 in controlling the size of captured individuals for 

 some species but not others, depending upon the 

 range in lengths of fish that a given mesh size 

 captures with high efficiency. For species where 

 the regulation can be useful (as indicated by low 

 values oiSs, ors,), the objective of the regulation is 

 usually to protect from harvest individuals of a 

 species below a certain length without decreasing 

 efficiency in the commercial gill net fishery. 

 Determination of the smallest mesh size that can 

 be fished is critical for the fish population and for 

 the fishermen. If the mesh size is too small, a 

 significant portion of the small individuals which 

 are to be protected will be caught. If the mesh size 

 is too large, the fishermen will possibly be pre- 

 vented from using a mesh size which would result 

 in high capture efficiency on legal-sized fish in the 

 population. Information presented in Tables 1 and 

 2 and Figures 1 and 2 can be used, with various 

 degrees of reliability, to evaluate the usefulness of 

 mesh-size regulations and, for some of the 22 

 species, to estimate the mesh size which would 

 best fulfill the above stated objective. 



At least small amounts of gill net selectivity 

 information were provided on 15 species (Table 1) 

 of fish that were caught and sold by commercial 

 fishermen along the south Atlantic and Gulf of 

 Mexico. The probability that the size composition 

 of the populations for some of these species will 

 eventually be controlled, partially by mesh-size 



regulations, is high. Of the 15 species, the sizes of 

 individuals caught by gill nets can be controlled, 

 possibly to a degree required for management 

 purposes, by mesh-size regulations, except for 

 bluefish and Spanish mackerel, based on the 

 available data. The degree of control, and the ef- 

 fect that a particular regulation would have on 

 capture efficiency for legal-sized fish in the fishery, 

 can be estimated from values of Ss, or s r 



Assuming that a mesh-size regulation is de- 

 sirable to manage a particular fishery, the steps in 

 estimating the "optimum" mesh size are as follows 

 for two examples — Atlantic croaker and Florida 

 pompano. These two species were selected as 

 examples because, for croaker, data were 

 sufficient to derive selectivity curves and, for 

 pompano, we had insufficient data to derive the 

 curves. 



1. Based on management objectives, determine 

 the maximum length (L) offish which you want to 

 protect from harvest ( minimum length offish to be 

 harvested) and the percent of catch allowed below 

 this length. We arbitrarily selected a length of 20 

 cm, and <2.5% as the maximum percent allowable 

 of fish below 20 cm, for each species. 



2. For Atlantic croaker, the slope (k) for the 

 equation relating mesh size (m,) and mean selec- 

 tion length (/,), and a weighted mean of the s, 

 estimates of the selectivity curves (Table 2) were 

 used to determine an estimate of the required 

 mesh size. The calculations follow: 



A. determine s = /£(«, + n l + l )s, 2 /^,n, = 1.56 



B. determine the minimum mesh size (mm;) 

 mm, = (L + 2s)lk = (20 cm + 3.1D/3.527 = 

 6.5 cm. 



Based on the above, one would expect about 2.5% 

 of the total catch to be composed of Atlantic 

 croaker under 20 cm total length by a gill net 

 having a stretched-mesh size of 6.5 cm. 



3. For Florida pompano, appropriate equations 

 to determine /, and s, are not available, because 

 selection curves could not be determined. These 

 values can be estimated, however, if we assume 

 that the empirical means and standard deviations 

 (SI, and Ss,; Table 1) are reasonable estimates of / ; 

 and s,. Estimates of the mean length-mesh size 

 relation and standard deviations based on the 

 above assumption would probably yield reason- 

 able and useful approximations for Florida 

 pompano, because: A) the length range within 

 which the pompano were caught efficiently in a 



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