Hart: Yield and biomass-per-recruil analysis of rotational fishenes 



51 



on those cohorts that reached exploitable size at about the 

 time the area was reopened, thereby resulting in only a 

 modest gain in yield-per-recruit. A more substantial gain 

 in maximum yield-per-recruit (up to 30% greater than 

 constant fishing) can be obtained if the closure is timed 

 to optimally exploit an unusually large year class. These 

 results are consistent with several studies that indicate 

 that periodic fishing can often increase yields over con- 

 stant fishing (Botsford, 1981; De Klerk and Gatto, 1981; 

 McCallum, 1988; Clark, 1990; Myers et al., 2000). 



A second, and perhaps more important, advantage of 

 rotational fishing is that it alleviates the impact of both 

 growth and recruitment overfishing. Growth overfishing 

 (i.e. fishing at a level higher than Fj^^j^) under rotational 

 management induces a substantially smaller reduction 

 in yield-per-recruit than would occur with constant fish- 

 ing. Rotation also increases biomass-per-recruit for sea 

 scallops, especially for levels of F above F^^y^, thereby 

 reducing the impact of possible recruitment overfishing. It 

 might be argued that overfishing should not be occurring 

 in any case. However, even when management measures 

 are taken to eliminate overfishing, it can still occur, for 

 example, if 1) reference points are incorrect because of un- 

 certainty in life history parameters; 2) fishing mortality, or 

 the effect of management measures on fishing mortality, 

 has been underestimated; or 3) there is localized overfish- 

 ing because of spatial variation in fishing intensities or 

 life history parameters (or variation in both), even though 

 when averaged spatially, F,^y(. < F^^^ (Caddy, 1975; Hart, 

 2001). Rotational fishing can thus be thought of as part of 

 a precautionary strategy. In so much as it may increase 

 maximum yield, rotational management is superior to 

 many other precautionary measures that reduce yield. 



The only costs of rotational management are the costs of 

 administrating and enforcing such a system, and socioeco- 

 nomic costs from temporary closures of traditional fishing 

 grounds. The latter might be significant if closures force 

 fishermen to make long distance steams to unfamiliar 

 areas. Because the optimal F^vg under rotation is only 

 slightly greater than the nonrotational F.^^^^^. the amount 

 of effort and fleet capacity required to optimize yield-per- 

 recruit under rotation is about the same as that needed 

 under uniform fishing. 



Rotation also imposes practical constraints on the level 

 of average fishing effort, thereby limiting the extent to 

 which stocks can be overfished. Fishing mortality rates for 

 U.S. sea scallop stocks were estimated as exceeding 1.0/yr 

 during the late 1980s and early 1990s. ^ This would corre- 

 spond under a 6-yr pulse rotation to an unaveraged fish- 

 ing mortality of over F = 6 in the area open to fishing. Such 

 a high fishing morality rate, corresponding to about a 98% 

 exploitation rate for fully recruited scallops, is likely to be 

 impractical for both physical and economic reasons. Thus, 

 F.^y^, in a rotation plan would likely be considerably below 

 the high levels seen in the late 80s and early 90s, even if 

 there was no other restriction on fishing effort other than 

 pulse rotation. 



Myers et al. (2000) claimed that "near-optimal yields 

 are achieved across a wide range of fishing mortalities" in 

 their rotational scheme. However, much of their analysis 

 was confounded by their use of unaveraged open area fish- 

 ing mortality (=pF^yq) on the x axis of their per- recruit 

 curves. For example, in the case analyzed in Myers et al 

 (2000), where one of p areas would be fished each year, 

 the fishing mortality F applied in the area open to fishing 

 in a 9-vr rotation (i.e. 1/9 of the area would be fished each 



