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Maximum sustainable yield and effective fisheries management

FishNet USA – January 2009

Nils E. Stolpe


 Due to ill-advised amendments to the Magnuson Stevens Fisheries Conservation and Management Act, all federally managed fisheries are required to be at a level that will produce the maximum sustainable yield. This is a requirement in spite of the fact that having “competing” species at this level might be biologically impossible or undesirable for economic or other reasons. The folly of this legislative mandate becomes obvious in an examination of the current situation regarding the “plague” (according to both recreational and commercial fishermen) of spiny dogfish, Squalus acanthias, currently impacting many fisheries from South Carolina to Maine.

In specific instances, and the dogfish situation off the Northeast coast provides a sterling example, fishing pressure can—and should—be used as an effective management tool. Given the basic fact that a particular area of ocean can only support a limited biomass of fish, by fully understanding and carefully controlling the makeup of that biomass through selective fishing, the species mix of the fish available for harvest can be optimized, producing a true optimum yield.

The Maximum Sustainable Yield (MSY) of a stock of fish is a theoretical level of harvest that will allow the stock to replenish itself continuously. Most simply, the rate of increase of the stock will be balanced by the rate of removal from the stock. When this condition is reached, the conventional wisdom has it, the fishery will be sustainable.

If any assumption can be said to be basic to modern fisheries management, it is that the sustainability of a fishery can be guaranteed by properly controlling fishing mortality. In fishery after fishery in which the population is not at a level that will produce the theoretical MSY, management efforts consist primarily of reducing fishing effort. Implicit in this is the belief that fishing is the most significant factor in determining if a fish stock is at the MSY level or not and is the only variable, that all other sources of fish mortality are negligible relative to fishing mortality and are constant as well.

Obviously this is not the case. On the macro-scale, regime shifts affecting entire ocean basins are accepted as regular occurrences. These profound perturbations have significant consequences for entire ecosystems and on virtually all of the fish stocks in them. On a lesser scale, non-fishing anthropogenic and natural factors can and do affect processes like spawning success and recruitment. These directly affect stock size. And most obviously, big fish eat little fish, so a bumper crop of species X can have a dramatic impact, positive or negative depending on who’s eating who, on species Y and Z. These phenomena aren’t necessarily regular, predictable or identifiable, yet in fisheries management they are treated as if they are, all being lumped together under Natural Mortality and assumed to be constant.

Maximum Sustainable Yield and the Magnuson Stevens Fisheries Conservation and Management Act -

Spiny dogfish – the poster fish for mismanagement

(from Commercial Fisheries News)


lbs Dogfish

lbs All Species

% Dogfish

Fall '06

69,031 161,234 43%

Spring '06

66,680 107,349 62%

Winter '06

58,943 114,605 51%

Total '06

194,654 383,188 51%

Fall '05

73,321 152,666 48%

Spring '05

46,992 83,465 56%

Winter '05

79,900 121,062 66%

Total '05

200,213 357,193 56%

Fall '04

58,923 145,430 41%

Spring '04

32,341 94,848 34%

Winter '04

89,932 150,237 60%

Total '04

181,196 390,515 46%

Fall '03

32,661 124,099 26%

Spring '03

55,654 133,134 42%

Winter '03

86,862 163,578 53%

Total '03

175,177 420,811 42%

Fall '02

33,668 153,542 22%

Spring '02

49,496 111,770 44%

Winter '02

88,233 164,748 54%

Total '02

171,397 430,060 40%

Fall '01

58,062 128,892 45%

Spring '01

26,321 75,564 35%

Winter' 01

91,686 186,301 49%

Total '01

176,069 390,757 45%

Fall '00

57,018 140,280 41%

Spring' 00

24,961 96,789 26%

Winter '00

45,923 91,674 50%

Total '00

127,902 328,743 39%

Fall '99

34,720 118,596 29%

Spring '99

36,434 87,783 42%

Winter '99

88,268 139,124 63%

Total '99

159,422 345,503 46%

Too much of a good thing?

How can yields actually be optimized?

How are the fish of the Northeast U.S. really faring?