Rotational management of sea scallops. in practice and in theory. Dvora Hart and Paul Rago NEFSC, Woods Hole

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1 Rotational management of sea scallops in practice and in theory Dvora Hart and Paul Rago NEFSC, Woods Hole

2 In December 1994, three large areas on Georges Bank were closed to trawling and dredging, primarily to protected overfished groundfish stocks The closed areas comprised about a half of the sea scallop productivity on Georges Bank A number of other restrictions (eg, ring size, DAS limits) came into effect in Closed Area I Closed Area II Nantucket Lightship Closed Area

3 Since the closures, substantial increases in scallop biomass and mean size has been observed in these areas Georges Bank Sea Scallop Density NMFS Annual Sea Scallop Survey 15 Overall density Closed area density Open area density B MAX Sea scallop density (kg/tow, meats) Year Area closures

4 US Georges Bank Sea Scallop Length-Frequencies Shell height (mm) /tow

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7 There is increasing evidence that the closures may have also improved recruitment, due to the large increase in SSB and perhaps also an Allee effect Region Mean Recruits Mean Recruits Georges Bank Mid-Atlantic 58 99

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9 In April, 1998, two areas in the Mid-Atlantic Bight were closed to scallop fishing for three years Unlike the Georges Bank closures, these areas designed to protect concentrations of small scallops in these areas Hudson Canyon Closed Area Virginia Beach Closed Area

10 Rapid increases in biomass were observed in these areas as well Mid-Atlantic Bight Sea Scallop Density NMFS Annual Sea Scallop Survey Overall density Closed area density Open area density B MAX Scallop density (kg/tow, meats) Year Area closures

11 Sea scallop size-frequencies Mid-Atlantic Bight survey mean Shell height (mm) /tow

12 The reopenings A portion of Closed Area II was opened to limited fishing in 1999, and about 6 million lbs of scallop meats were landed Portions of all three areas were reopened for brief periods in 2000, landing about 5 million lbs Despite this, scallop biomass in the groundfish closed areas has continued to increase In 2001, the Mid-Atlantic closed areas were reopened, with the Georges Bank areas closed To date, over 7 million lbs have been taken from the Mid-Atlantic closed areas By drawing effort away from the open areas, which remain overfished, these reopenings have contributed to the rebuilding of scallop biomass in the open areas

13 Longitude L a ti t ud e Longitude Longitude L a ti t ud e Longitude L a ti t ud e Oct 1998

14 Longitude L ati tu d e Longitude Longitude Longitude Oct 1999

15 Exploitable biomass and commerical LPUE Commercial LPUE (lbs/day) Exploitable biomass - open areas (g/tow) Effort reduction and redirection, gear restrictions, and good recruitment have had dramatic effects on both open area survey biomass and commercial LPUE LPUE Mean LPUE, Expl Bms, open areas Year

16 Conclusions, Part I Area closures have been shown to be an effective way to rebuild sea scallop biomass, alleviate growth overfishing, and probably also improve recruitment These experiences suggest the idea of rotational fishing for sea scallops

17 Part II: Some aspects of the theory of rotational fishing Sea scallops are an ideal candidate for rotational fishing because: (1) They have low mobility, probably moving at most a few nm/y (2) They are long-lived, with low natural mortality (M ~ 01) (3) They grow quickly (K ~ 03) (4) They are selected by the gear well before they are of optimal size

18 Yield-per-recruit analysis for mechanical rotations Unlike ordinary yield-per-recruit analysis, different cohorts experience different fishing mortality patterns For example, for a four year rotation with 3 years closed and 1 year open, a cohort can experience one of the following four mortality patterns: CL CL CL OP CL CL CL OP or OP CL CL CL OP CL CL CL or CL OP CL CL CL OP CL or CL CL OP CL CL CL OP CL

19 Over a whole period, any of these cohorts need to be fished at 4F in the one year it is fished in order to produce the same fishing mortality rate as constant fishing at F More generally, fishing at F 1, F 2,,F n produces the same mortality risk over this n year period as constant fishing at: F AVG = (F 1 + F 2 ++F n )/n In other words, fishing mortality must be temporally averaged

20 The sequence of zero and high mortality rates means that different cohorts experience different mortality patterns and give different levels of YPR YPR (g) 5 0 Average cohort Worst yielding cohort Best yielding cohort F AVG (y -1 )

21 When YPR is averaged over all cohorts, a regular rotation gives only a slight advantage over uniform fishing at F MAX Rotation does do better at high fishing mortalities because it allows some individuals to grow to a large size, no matter how high the mortality rates YPRAVG (g) No rotation 3 yr rotation 6 yr rotation 9 yr rotation F AVG (y -1 )

22 Adaptive rotation Further improvements in yield can be made by timing closures so as to benefit large year classes Such an approach requires a simulation model to simulate localized recruitment, growth and mortality

23 Simulation/Forecasting Model Log-normal stochastic recruitment in each subarea, with means and covariances matching the area-specific time-series Von Bertalanffy growth Area-specific fishing mortality, depending on the given management scenerio and on a fleet dynamics model Non-linear saturating biomass/lpue model (modified Holling type II) used to relate fishing mortalities to DAS

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26 Control Rules for Adaptive Rotation Areas are closed when its growth rate exceeds a given threshold Closures last a fixed amount of time (3-5 y), or until the growth rate is reduced below a second threshold When an area is reopened, it is subject to a high F for three years, so that the time-averaged fishing mortality for the period since the closure began is equal to the target F (02) The total amount of biomass closed is subject to a cap Best results are obtained for low growth closure thresholds (15-20%), 3 year closures, and 25% maximum biomass closed

27 Uniform fishing Fixed rotation Adaptive rotation Temporally averaged fishing mortality YPR (g)

28 Area closures require a rethinking of conventional fishing mortality metrics: (1) Spatial averaging is inappropriate - high biomass in closed areas can mask growth overfishing in open areas For example, since about 80% of scallop biomass in Georges Bank is in the closed areas, the overall fishing mortality rate cannot exceed the target F, even if the mortality in the open areas is well beyond that which gives maximal YPR (2) Fishing mortality in areas that had been recently closed needs to be higher than the conventional reference point in order to maximize YPR - fishing mortality needs to be temporally averaged

29 Conclusions Area closures and rotational fishing can be an useful way of rebuilding and managing sedentary stocks such as sea scallops However, rotational management requires a rethinking of conventional notions of fishing mortality, and needs to be applied adaptively to be most effective