Aireguin-Sanchez and Pitcher: Catchability estimates for the Epmephelus mono fishery 



751 



Time-dependent catchability 



Time variation of catchability was analyzed 

 considering both annual and monthly catch- 

 structured data. From Equation 7, parameters 

 (Ay) and /3(y) were estimated by regressing 



Ln[i7(/,n/t/(/,.)] 



with length (Fig. 2). Table 3 shows values of 

 the constants for both midsize Mexican and 

 Cuban fleets for annual data. 



On a monthly basis, the same computa- 

 tions were developed for the mid-size fleet. 

 Departures represented in parameters cAm) 

 and l^km ) were estimated in a similar way to 

 those in Equation 7, but considering annual 

 catch-structured data. Seasonal changes in 

 \'ulnerability occurred: q increased in win- 

 ter (December to February) in synchrony 

 with the reproductive aggregation (Fig. 3). 

 Catchability was lower in autumn, when the 

 population is dispersed along the continen- 

 tal shelf and the vulnerability of young fish 

 in coastal waters increases. 



Catchability differences between fleets 



Following Equation 11, differences between 

 fleets were evaluated by comparing annual 

 data for the Cuban fleet with those of the mid- 

 size fleet for the period 1975 to 1987. Mexi- 

 can mid-size and artisanal fleets were com- 

 pared on a monthly basis for 1987. Param- 

 eters for a(f) and fiif) were obtained (Tables 

 4 and 5). Positive values o^ fHf) indicate that 

 relative catchability for small fish is higher 

 for the midsize fleet than for the Cuban fleet; 

 and lower for large fish. This scheme repre- 

 sents differences in fishing areas between 

 fieets. The negative value jiif) for in 1982 

 suggests fieets changed their usual behav- 

 ior. Similar interpretation can be made for 

 monthly data between the artisanal and mid- 

 size fieets. 



Figure 3 shows the tendency of 



Ln[f/ (mid-scale, /,n /[/(Cuban, /,n] 



ra 



CO 

 O 



2 ' 

 Ot 

 -2 - 

 ■A - 



2 - 



2 - 



1 



- 



-1 - 



2 



- 



-2 - 



-A - 



^juj,, ,jjsrrf<^''°^ -' \juo^ - ^ ~'->'- ^ ^ 



g^ ^j; ncxxmnnnnocxx)oc xxoou uuuu u o " c^ 



1973 



1975 



1977 



„oO 



,0^/ - '='='°°°'^'^°°°^^^ 



-^cneeGOOQQDQooooeeeGo 



1979 



1981 



c^fOr^x j yy:frnMxijJ''^''^'^^ 



^p : :r^rrx^^C)QiJu^J^J: : px>D^^ 



i HgPOrrooorxJoocxiocujo u u u utx/^ 



oo 



^ 



1983 



1985 



1987 



20 40 60 80 100 



Total Length (cm) 



Figure 2 



Annual tendencies of the ratio Ln[f/(', t}/U{ ',•)! with length for 

 the Mexican midsize fleet of the red grouper fishery representing 

 departure of catchability-at-length pattern with respect to equilib- 

 rium, as expressed in Equation 6. 



with length. An infiection point is observed 

 just at the length at first maturity (i.e. 50 

 cm LT). Positive values indicate that q is 



higher in small (immature) fish for the midsize fieet 

 than for the Cuban fieet, whereas q is equal in both 

 fieets for mature fish. For the tendency of 



Ln[[/(artisanaU.n/[/( mid-scale, ^n] 



with length, catchability of immature fish was higher 

 for the artisanal than for the midsize fieet, and again 

 equal for adult fish up to 65 cm TL. For fish larger 

 than 65 cm, q was higher in the midsize fieet. 



