TRENT and PRISTAS: SELECTIVITY OF GILL NETS 



particular mesh size was narrow; B) they rarely 

 became entangled in the webbing; and C) a wide 

 range of sizes was available in the fished popula- 

 tion (Table 1 ). Based on the above assumption, the 

 equations are: 



A.Ss 

 B. mm. 



= v Infis^ln, =3.12 



based on data where n,>9 and 

 = (L + 2Ss)/Sk ---- (20 cm + 6.24V2.517 

 = 10.4 cm 



where Sk = the slope of the least squares regres- 

 sion line fitted through the origin to the points 

 shown in Figure 1 for Florida pompano. Thus, 

 2.57c of the catch of pompano in gill nets with mesh 

 size of 10.4 cm can be expected to be below 20 cm in 

 length. 



Capture Efficiency 



Several factors should be considered in the 

 selection of mesh sizes for maximizing the ef- 

 ficiency of capture. Efficiency of capture is defined, 

 or measured by, the dollar return per unit of effort 

 in a gill net fishery. In a gill net fishery the more 

 important factors include: 1) whether individuals 

 of a single species or a group of species are sought; 

 2) the regulations (mesh size, minimum size limit, 

 etc.) that exist in the fishery; 3) how the gill net is 

 to be fished (anchored, drift, run-around, etc.); 4) 

 values of the species sought and values of 

 various-sized individuals in the fished popula- 

 tions; 5) information on the life history of each 

 species sought, especially the mean length of each 

 age class, the variation in year-class strength 

 between years, and the length-weight relation; 6) 

 the ability, in terms of cost, to use nets with more 

 than one mesh size; and 7) the most efficient mesh 

 sizes for capturing various lengths of fish in the 

 fished population. For this discussion the only 

 factor to be considered is the determination of 

 efficient mesh sizes. 



For the 15 species of fish of commercial im- 

 portance shown in Table 1, the efficiency of cap- 

 turing a particular length group with maximum 

 efficiency is highly dependent on mesh size for all 

 except bluefish and Spanish mackerel. The range 

 in lengths offish that a particular mesh size would 

 capture with high efficiency can be estimated from 

 values of s, or Ss t given in Tables 1 and 2. The 

 equations, 



I sl, 



m, =-orm, = — 



similar to those in the previous section, and with 

 the same reservations regarding the accuracy of 

 the estimates, can be used to estimate the most 

 efficient mesh sizes for capturing various lengths 

 offish. A discussion of this type of application in a 

 particular fishery was given by Trent and Hassler 

 (1968). 



Limitations on Uses 



Selectivity information derived for the 10 

 species in this study as shown in Figure 1 should 

 be used cautiously, if at all, in adjusting length- 

 frequency distributions. The assumption that the 

 shapes and amplitudes of the selectivity curves 

 are the same for a species could not be tested, but is 

 probably not valid (Hamley and Regier 1973). 

 Further, for all species except Atlantic croaker 

 and blue runner to which we have applied Holt's 

 method, one or more of the three assumptions were 

 invalid, or questionable, or sufficient data were 

 not available to evaluate the assumptions. 



Several other factors, not investigated in this 

 study, should be considered when applying our 

 results to estimate mesh sizes for controlling 

 capture efficiency or in adjusting length- 

 frequency distributions of the catch. Selection is 

 dependent to some extent on factors other than 

 mesh size. We used set gill nets, all of which were 

 constructed in the same manner from one type of 

 webbing material. Fishing often occurs with gill 

 nets by encircling the schools or by blocking an 

 area and scaring the fish into the net, or waiting 

 until falling tides force the fish from the blocked 

 area. When fishing is conducted in these ways, 

 many individuals are often caught loosely wedged 

 ( Garrod 1961 ) or loosely entangled in the net; most 

 of these fish, if set gill nets had been used, would 

 have eventually escaped. Selection (size of cap- 

 tured individuals, or efficiency of capture, or both) 

 is also dependent on other factors: natural or 

 synthetic webbing (Washington 1973); color of 

 webbing (Jester 1973); twine size (Hansen 1974); 

 and the hanging coefficient (Hamley 1975). 



LITERATURE CITED 



BRUSHER, H. A., AND L. H. OGREN. 



1976. Distribution, abundance, and size of penaeid 

 shrimps in the St. Andrew Bay system, Florida. Fish. 

 Bull., U.S. 74:158-166. 

 CUCIN, D., AND H. A. REGIER. 



1966. Dynamics and exploitation of lake whitefish in 

 southern Georgian Bay. J. Fish. Res. Board Can. 

 23:221-274. 



195 



