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



47 



No rotation 

 3 yr rotation 

 6 yr rotation 

 9 yr rotation 



04 

 F (/yr) 



Figure 1 



Yield-per-recruit curves for Georges Bank sea scallops with no rotation and with 3- 

 6-. and 9-yr pulse rotations. 



rotations and higher fishing mortalities. At F[^i,\x- 

 the increase in biomass-per-recruit is shght, un- 

 less a very long (e.g. 9-yr) rotational period is 

 employed. 



The performance of rotation can be assessed as 

 a function of selectivity at size. Figure 5A gives 

 maximal yield-per-recruit for a number of pulse 

 rotation strategies and a variety of values of h^^^^, 

 the smallest size selected by the gear; the size of 

 full selectivity, /if^,,, was taken as h^^^^ + 23 mm 

 (consistent with the assumed current gear selec- 

 tivity pattern; see Table 1). Rotation can give sub- 

 stantial yield-per-recruit advantages when the 

 gear selects animals of well below optimal size, 

 especially for longer periods. However, long-pe- 

 riod rotation actually gives less yield-per-recruit 

 than constant fishing for larger values of h^^^. 

 Figure 5B gives a similar plot for biomass-per- 

 recruit, where fully recruited fishing mortality 

 is fixed at F = 0.3 in all cases. Unlike yield-per- 

 recruit, rotational fishing increases biomass-per- 

 recruit regardless of the selectivity pattern, espe- 

 cially when the rotational period is long. 



Yield-per-recruit from difTerent cohorts under 

 a rotational system can vary considerably. The 

 cohort which recruits into the fishery at about the time of 

 the closure produces the highest yield-per-recruit, whereas 

 the cohort that reaches exploitable size at about the time 

 that the area is opened has the lowest. Figure 6 gives the 

 mean yield-per-recruit together with that of the cohorts 

 with the highest and lowest yield-per-recruit under six- 

 year pulse rotational management (i.e. i^^vc^ ^"'' ^""^ ^*'' 



• • 



• • 



o o 



o o o 



2 4 6 8 10 



Rotation period p(yr) 



Figure 2 



Maximum yield-per-recruit (solid circles) and discounted (Wv) 

 yield-per-recruit (open circles) for Georges Bank sea scallops with a 

 pulse rotation of periods between 1 and 1 1 years. 



respectively). A 319^ increase in maximal yield compared to 

 constant fishing (and 25'7c increase over the average yield- 

 per-recruit under rotation) can be obtained from the cohort 

 whose yield-per-recruit is the highest under rotation. Note 

 that, unlike conventional yield-per-recruit cun'es for sea 

 scallops, yield-per-recruit from this cohort is almost com- 

 pletely insensitive to effort beyond a certain level. 



