52 



Fishery Bulletin 101(1) 



year and that F = F^yc, ^9' would represent one ninth of 

 the actual effort as the same F applied to the whole area 

 under non-rotational fishing. Use of unaveraged fishing 

 mortality has the effect of stretching the .v axis by a factor 

 of p, thereby making their graphs appear flatter than they 

 actually are. F^vg ^^ representative of not only the true 

 time-averaged fishing mortality but also in many cases 

 would be proportional to spatially averaged fishing effort 

 (as measured by, e.g., hours fished). 



Myers et al. (2000) also suggested that rotational fishing 

 would help lessen the impact of indirect (incidental) fish- 

 ing mortality on yield-per-recruit. The analysis given in 

 the present study indicates that incidental mortality low- 

 ers yield-per-recruit at /^max about the same amount re- 

 gardless of whether or not rotational fishing is employed. 

 At levels of fishing mortality well above F^i^y^, rotational 

 fishing does appear to modestly decrease the loss of yield- 

 per-recruit due to incidental mortality. This decrease is 

 due to the fact that incidental mortality, by somewhat 

 lowering F^^^, exacerbates the effects of overfishing, 

 whereas rotation alleviates the loss of yield-per-recruit 

 due to overfishing. 



The effectiveness of sea scallop rotational fishing can be 

 understood by examining fishing mortality at size for vari- 

 ous rotational strategies. F'igure 7 shows fishing mortality 

 as a function of shell height for no rotation and for 3-, 6-, 

 and 9-yr rotations for F^^q = 0.2 (Fig. 8A), and /-^vp = 0.6 

 (Fig. 8B). Rotational fishing (especially for longer periods) 

 tends to reduce the fishing mortality on small scallops 

 and shift this effort onto larger individuals, thereby in- 

 creasing yield-per-recruit, especially when overfishing is 

 occurring. The periodic peaks in fishing mortality seen 



in the rotational strategies occur at the sizes where a 

 new cohort begins to be fished (i.e. when the scallops are 

 some integer number of years past their age at 40 mm). 

 In practice, these peaks are likely to be much less pro- 

 nounced because of variations in individual gi'owth rates 

 and settlement times. However, the qualitative pattern of 

 increasing selectivity with size should not be affected by 

 such variations. 



Sea scallops are an ideal candidate for rotational man- 

 agement, combining fast growth and low natural mortality 

 with a sedentary adult lifestyle. In addition, sea scallops 

 are recruited into the fishery at a size that is well below 

 optimal from a yield-per-recruit perspective. The increase 

 in size-selectivity induced by rotation described above 

 should therefore induce an increase in yield-per-recruit. 

 However, in those fisheries where the size-at-entry to the 

 fishery is much larger, rotation would not be expected to 

 induce gains in maximal yield-per-recruit (see Fig. 5A). 

 On the other hand, it appears that rotation increases 

 biomass-per-recruit regardless of the size-selectivity of 

 the fishery (Fig. 5B). Botsford et al. ( 1993) found that rota- 

 tion increased biomass- but not yield-per-recruit for red 

 sea urchins. These results are consistent with the above 

 discussion because the minimum legal size for landing the 

 urchins was already near-optimal. 



Although the exact levels of yield- and biomass-per- 

 recruit obtained with or without rotation are sensitive 

 to such factors as natural mortality and growth rates, 

 the relative gains of rotation over constant fishing are 

 much less sensitive to these factors. Rotation will improve 

 yield-per-recruit under a broad range of parameter choices 

 provided that li the ratio of growth to mortality K/M is 



