Hart: Yield- and biomass-per-recruit analysis of rotational fisheries 



53 



0,30 



025 



020 



15 - 



0,10 - 



0-05 - 



0.00 



1.2 



1.0 



0.8 



0.6 



0.4 



0.2 - 



0.0 



No rotation 

 Ttiree year rotation 

 Six year rotation 

 Nine year rotation 



80 



No rotation 

 3 yr rotation 

 6 yr rotation 

 9 year rotation 



100 



120 



140 



80 



100 120 



Stiell tieight (mm) 



140 



Figure 8 



Mean fishing mortality at length for Georges Bank sea scallops with no rotation 

 and with 3-. 6-. and 9-yr pulse rotations for (A) F = 0.2, and (B) f = 0.6. 



sufficiently high (greater than about 0.5 with the other 

 parameters in the model fixed as given in Table 1), and 2) 

 size-selectivity is suboptimal. Rotation improves biomass- 

 per-recruit under even a wider range of parameters. 



Allee effects may occur in broadcast spawners such as 

 urchins and scallops. Areas that are closed for several 

 years may allow these animals to form dense aggregations 

 (that would likely be heavily fished if not closed), thereby 

 improving fertilization success (Botsford et al., 1993). 

 Such an effect would mean that rotation could produce 

 greater benefits in fecundity than would be suggested by 

 biomass- or eggs-per-recruit curves. 



Metapopulation structure might also be considered 

 when designing a rotational strategy. If recruitment is 

 limited by the supply of settling larva, an area that is a 

 source of larva might be fished less than that required to 

 maximize yield-per-recruit in order to increase larval sup- 

 ply (Tuck and Possingham, 1994). 



The calculations that indicate long optimal rotational 

 periods assume low constant natural mortality, indepen- 

 dent of age or density, based on the study of Merrill and 

 Posgay (1964). There is some evidence that the natural 

 mortality rate of sea scallops may increase with age or 

 size for shell heights greater than about 110 mm (Mac- 



