SELECTIVITY OF GILL NETS ON ESTUARINE AND 

 COASTAL FISHES FROM ST. ANDREW BAY, FLORIDA 



Lee Trent and Paul J. Pristas 1 



ABSTRACT 



Eleven gill nets, each of a different mesh size, were fished 126 days from 4 April to 29 December 1973 in 

 St. Andrew Bay, Fla. Of the estuarine and coastal fishes that were caught, 22 were in numbers 

 sufficient to evaluate the relation between length offish and mesh size. Mean length increased with an 

 increase in mesh size for 20 species. Ten species — gulf menhaden, Brevoortia patronus; spot, Leios- 

 tomus xanthurus; sea catfish, Arius felis; pinfish, Lagodon rhomboides; Atlantic croaker, Micropogon 

 undulatus; blue runner, Caranx crysos; pigfish, Orthopristis chrysoptera; bluefish, Pomatomus sal- 

 tatrix; Spanish mackerel, Scomberomorus maculatus; yellowfin menhaden, B. smithi — were caught in 

 sufficient numbers to apply and evaluate the normal probability model to define gill net selectivity. 

 One or more of the three assumptions — normality of selectivity curve, linearity of mean length-mesh 

 size relation, and constancy of standard deviation between mesh sizes — inherent in the model was 

 violated by the data for each species to which the model was applied except Atlantic croaker and blue 

 runner. Useful information was provided, however, in relation to evaluating mesh-size regulations and 

 for determining mesh sizes for increasing capture efficiencies in gill net fisheries. 



Rarely will a particular type of fishing gear cap- 

 ture all sizes of a species of fish with equal prob- 

 ability. Gill nets are selective in that, for a par- 

 ticular species and mesh size, fish are retained 

 with high probability at certain lengths and with 

 decreasing probability for larger and smaller 

 individuals. Most streamlined fish without pro- 

 jecting spines, teeth, or opercular bones are caught 

 in gill nets by becoming tightly wedged or en- 

 meshed in the webbing. To describe selectivity for 

 these streamlined fishes, a smooth unimodal curve 

 with capture probabilities descending to zero is 

 suggested by several workers (Regier and Robson 

 1966). Fish species that are not streamlined, or 

 that have stiff projecting appendages or spines, 

 are frequently caught entangled in the webbing 

 rather than, or in addition to, becoming wedged in 

 the meshes. For these species skewed or multi- 

 modal curves are usually necessary to describe 

 capture probabilities (Hamley and Regier 1973). 

 An understanding of the selection properties of 

 gill nets is necessary to evaluate catch statistics, 

 alter catch per unit effort, and regulate the sizes of 

 caught fish. Most methods of estimating re- 

 cruitment, growth, sex ratio, and survival of a fish 

 species require samples that are representative of 

 the population in respect to size of individuals. 



'Southeast Fisheries Center Panama City Laboratory, Na- 

 tional Marine Fisheries Service, NOAA, P.O. Box 4218, Panama 

 City, FL 32401. 



Only if size selectivity of the fishing gear is known 

 can the catch statistics be adjusted and used to 

 provide correct estimates of the parameters of 

 interest (Cucin and Regier 1966). Alternatively, 

 an understanding of how selectivity depends on 

 the characteristics of the gear may be used to de- 

 sign a series of gear to yield samples of known 

 characteristics over a specified size range (Regier 

 and Robson 1966). A knowledge of the size selec- 

 tive properties of the gear permits recommen- 

 dations of mesh sizes to maximize (increase cap- 

 ture efficiency) or minimize (protect from harvest) 

 the catch on certain sizes and species. 



Published information is not available on the 

 lengths of fish caught in particular mesh sizes of 

 gill nets for estuarine and coastal fishes inhabit- 

 ing the Gulf of Mexico except for a meager amount 

 on two species. Klima (1959) reported length- 

 frequency distributions of Spanish mackerel, 

 Scomberomorus maculatus, that were caught in 

 7.9- and 9.0-cm stretched-mesh gill nets. Modal 

 lengths of those were 37 and 43 cm, respectively. 

 Tabb (1960) reported a length-frequency dis- 

 tribution of spotted seatrout, Cynoscion 

 nebulosus, that were caught in 8.0-cm stretched- 

 mesh gill nets. Modal length of the distribution 

 was 33.5 cm. 



Mesh sizes of gill nets most frequently used to 

 capture various species of fish in the commercial 

 gill net fishery in Florida were reported by 

 Siebenaler (1955). 



Manuscript accepted August 1976. 

 FISHERY BULLETIN: VOL. 75, NO. 1, 1977. 



185 



