12 



Fishery Bulletin 103(1) 



Discussion 



Scup 



The type of net (gear) and the size of codend mesh had 

 only a minor effect on the length frequencies of scup 

 caught. Although variations in codend mesh size nor- 

 mally influence catch in other studies (Hastie, 1996; 

 Petrakis and Stergiou, 1997; Stergiou et al., 1997; Broad- 

 hurst et al., 1999), a wide range in codend mesh sizes 

 produced similar results for scup. Codends with some 

 meshes al2.7 cm appeared to catch more of the size 

 classes of fish chosen for landing than the composite 

 codend and just slightly more than the legal 11.43-cm 

 mesh codend; therefore the al2.7-cm mesh condend may 

 reduce discards. The actual difference in scup lengths 

 between the three codends was only about one cm. In 

 terms of kilograms caught, more scup were caught in 

 tows with the larger codend mesh. Landings increased, 

 but so did discards, so that the discards-to-landings ratio 

 remained unchanged. This finding indicates that the 

 small upward bias in sizes caught did not significantly 

 reduce total catch. In general, the smaller mesh codends 

 (6.35-10.16 cm) and the composite codend (10.16+11.43 

 cm) performed similarly to the current legal mesh design 

 (11.43 cm). Overall, discards of scup remained high 

 regardless of the type of gear (nets) and codends used. 



In our study, more larger scup were caught in longer 

 tows. When a boat encounters a large school of scup, 

 the mean length of the catch tended to be smaller. In 

 addition, the larger-size scup tended to be caught more 

 often in those tows with total catches below the me- 

 dian. This trend is probably a biological effect, but an 

 effect of mesh size or gear cannot be excluded. Most 

 populations contain relatively few larger fish and, there- 

 fore, more smaller individuals. Morse 6 (in Steimle et 

 al., 1999) noted that scup schools are size-structured. 

 When larger scup are less common in schools, then 

 schools with these larger individuals most likely would 

 be smaller and more effort would be required to achieve 

 the same catch of these individuals. The same result 

 would occur if larger scup tended to be on the outside 

 or above smaller scup in schools. Little is known about 

 scup behavior. However, any spatial size structure in 

 the population could promote a direct relationship be- 

 tween effort and the mean length of fish caught and an 

 inverse relationship between total catch (all species) 

 and mean length of fish caught. 



As an alternative explanation for the lower catch rate 

 of larger individuals, clogging of the codend may occur 

 when catch rates are high and, as a consequence, size- 

 selectivity would decline. Different codend mesh sizes 

 do not seem to affect the number of discarded scup as 

 much as one might anticipate because codends clog dur- 



6 Morse, W. W. 1978. Biological and fisheries data on scup, 

 Stenotomus chrysops (Linnaeus). NMFS, NEFSC, Sandy 

 Hook Lab. tech. ser. rep. no. 12, 41 p. James J. Howard 

 Marine Sciences Laboratory, Northeast Fisheries Science 

 Center, 74 Magruder Rd., Sandy Hook, NJ 07732. 



ing the interception of large schools. Accordingly, lower 

 CPUE could produce greater size selectivity resulting in 

 increased mean length when catches are relatively low. 

 However, the trends observed in length frequency with 

 effort and total catch were not significantly influenced 

 by codend mesh size. Accordingly, the observed trend is 

 likely a direct consequence of fishing on size-structured 

 populations. 



In general, more scup were landed and discarded in 

 the last half of a trip. This finding indicates that the 

 captain learns where to fish for scup by the second half 

 of the trip and CPUE increases as a consequence. We 

 had expected an increase in discards as the trip limit 

 was reached towards the end of the trip. However, no 

 effect of trip limits on the total weight of discards could 

 be discerned in our data set or the NMFS observer 

 data set. 



More scup were discarded per tow in tows observed 

 during the first half of the 2001 season, namely 1-24 

 January, than in the second half of the season, 25 

 January-February, but the discards-to-landings ratio 

 did not vary for either half of the season. The fact that 

 the ratio did not differ indicates that more scup are 

 discarded per tow when fishermen are allowed a larger 

 trip limit (4536 kg). The higher discards of scup per tow 

 during the first half of the season are likely due to the 

 increased total catch per tow that might be anticipated 

 when allowable landings are higher. Accordingly, cap- 

 tains are able to reduce catch rate and, thus, discards 

 when landing limits are low. 



We compared the NMFS observer database to our 

 observer data. Despite a substantial variation in the 

 distribution of codend mesh sizes between the two data 

 sets, the discards-to-landings ratio was not significantly 

 different. Concerns raised by the high discards-to-land- 

 ings ratio observed in the NMFS observer data were 

 supported by our study. The discards-to-landings ratio 

 for the directed scup fishery consistently exceeded 1.0. 



In summary, the objective of our study was to evalu- 

 ate the effect of codend mesh size on the amount of scup 

 discards and to identify mechanisms to reduce scup 

 discards. Although we observed a number of trends in 

 discards in our study, neither the current legal mesh 

 nor any of the experimental codends seem to adequately 

 filter out scup smaller than 22.86 cm. Neither did trip 

 limits seem to influence the total weight of scup dis- 

 cards. In fact, the only consistent trends produced by 

 variations in effort and total catch seem most likely 

 due to biological effects not easily controlled for by the 

 captain of a fish vessel. Overall, the total weight of dis- 

 cards seems to be primarily a function of the regulated 

 size limit, abetted by the tendency for smaller fish to be 

 captured when encounter rates are high. The present 

 study found that the length of the median discards was 

 about 17.78 cm FL (19.83 cm TL based upon a conver- 

 sion factor of Hamer 4 [in MAFMC, 1996]). O'Brien et 

 al. (1993) and NEFSC (1993) reported that 50% of both 

 male and female scup reach maturity at 15.49 cm FL 

 (17.27 cm TL). Therefore, lowering the scup minimum 

 size limit to 17.78 cm FL (19.83 cm TL) would greatly 



