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



55 



Figure 9 



Whole-stock fishing mortality as a function of efTort (F^vg' ''"' ^^^ scallops 

 with a 6-yr pulse rotation (solid line), and constant fishing with 10% of the 

 area permanently closed (dashed line). The dotted line is the line y = x. 



The current situation for sea scallops in Georges Bank 

 gives an even more extreme example of this phenomenon. 

 About 80% of the biomass lies in the groundfish areas that 

 have been closed to scallop fishing for most of the time 

 since December 1994. Because these areas will be closed 

 to scalloping in 2002, the whole-stock fishing mortality in 

 this year cannot exceed the F^y^ reference point of 0.24. 

 Therefore, according to the current overfishing definition 

 (the whole-stock F is below F-^j^x'- ^^^ stock cannot be 

 overfished. Nonetheless, the fishing mortality in the open 

 areas may exceed F^i^x- resulting in growth overfishing 

 in these areas. Thus, the stock in the open areas could be 

 overfished from a yield-per-recruit perspective even if the 

 whole-stock F is below F^y^. 



The opposite situation could also occur If scallops in the 

 groundfish closed areas on Georges Bank were fished more 

 than slightly above the Ff^^j^ = 0.24 reference point, the 

 whole-stock fishing mortality would also be above this ref- 

 erence point and overfishing would be considered to be oc- 

 curring. However, an area that has been closed for a number 

 of years should be fished harder, compared to an area that 

 has never been closed, once the area is reopened in order 

 to maximize yield-per-recruit. Thus, a strategy that would 

 maximize yield-per-recruit might require a whole-stock 

 F that would in some years be higher, and in some years 

 lower, than the conventional overfishing reference point. 



A whole-stock fishing mortality rate may therefore not 

 be the most appropriate metric for overfishing definitions 

 when some areas are temporarily or permanently closed 

 to fishing. Its value may not be representative of the yield- 

 per-recruit that could be obtained at that level of fishing 

 mortality. Furthermore, when most of the biomass is in 



closed areas, estimated whole-stock fishing mortality may 

 be more sensitive to variations in recruitment and mea- 

 surement error than to actual changes in effort. 



As an alternative to a whole-stock fishing mortality 

 metric, the following considerations are suggested for a 

 fishing effort measure that is compatible with yield-per- 

 recruit calculations. (1) Stock from areas that are not 

 fished in a given time period should not be included in 

 the fishing mortality calculation for that time period. In a 

 relatively sedentary stock, the amount of biomass in the 

 closed areas is irrelevant in determining the yield-per- 

 recruit that will be obtained from the stock in the open 

 areas. (2) Time-averaging of fishing mortality in the open 

 areas is required to take into account the previous fish- 

 ing history of the area. An area that has been closed for 

 a number of years needs to be fished harder once opened 

 than an area that has been continuously fished in order to 

 maximize yield-per-recruit. 



Based on these considerations, the time-averaged fish- 

 ing mortality computed from the open areas only, F.wg' 

 is an appropriate measure of fishing mortality in fisher- 

 ies managed by using rotational or indefinite closures. 

 It is natural to take the averaging period equal to the 

 rotational period p. With this metric. Fy^,^-^ is only slightly 

 sensitive to the rotational period p and completely insensi- 

 tive to the level of closed area biomass. Indeed, even if no 

 closures existed, but fishing effort varied with time, it may 

 still be advisable to employ a time-averaged fishing mor- 

 tality because the previous history of fishing mortalities 

 strongly affects the level of effort required to maximize 

 future yield-per-recruit. If an area has been fished harder 

 than Fm.yx for a number of years, so that the population 



