Dolphins and the Tuna Industry (1992) / Chapter Skim
Currently Skimming:
6 Dolphin Mortality and Abundance
6 Dolphin Mortality and Abundance
Pages 52-71
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From page 52... ...
. Methodological considerations, factors affecting total mortality estimates, and estimates for 1959-1990 are covered.
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From page 53... ...
· Compute the average mortality per day at sea and extrapolate to total fleet days. Days at sea and fishing activity correlate well, but the correlation is far from perfect, because days at sea are spent in activities other than fishing, such as running to the fishing grounds, waiting for storms to pass, or drifting because of a malfunction.
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From page 54... ...
The repetitions allow the calculation of standard errors without making a set of assumptions about the distribution of the variables. In addition to being useful for estimating variances and confidence intervals, the bootstrap procedure may reduce bias in the estimates, and for these reasons it was introduced into the estimation methodology.
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From page 55... ...
FACTORS AFFECTING TOTAL MORTALllY ESTIMATES Fishing Effort on Dolphins The catches resulting from school fishing, log fishing, and dolphin-associated fishing usually are quite different in species composition and distribution of sizes of fish caught. Sets on floating objects (log sets)
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From page 56... ...
See text. Data from IA11C; except total mortality data for 1972-1978; In those cases, the high range is from NMFS (1980a)
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From page 57... ...
Dolphin Mortality and Abundance 1 00% 75% 50% 25% 0%— 100% 75% 50% 25% 0% 57 . '/ /1 ~ / /1 79 80 81 82 83 /// V// 84 85 86 87 88 89 Year / , 79 80 81 82 Set Type 83 84 85 86 Year Dolphin sets flog sets G school sets 90 //7, 87 88 89 90 FIGURE 6-2 Proportion of sets (upper panel)
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From page 58... ...
The changes in oceanographic conditions, especially the patterns of the temperature fields and thermocline depth, may have driven the tuna deeper and made them less available to surface gear, or perhaps the changes in currents shifted the location of the productive fishing grounds to areas not normally fished by the fleet. Finally, due to a series of environmental, biological, and fishery factors, the abundance of large yellowfin increased to very high levels in the mid to late 1980s (IA11C, 1989a)
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From page 59... ...
Dolphin Mortality and Abundance 25 20 oo c ._ as Q 15ce o A, 1 0 3 At 5- ~ 0 59 ~ 1 ~ 0-1 2 S 4 6 ~ 7 8 9 10 11 12 13 14 16 16 17 18 19 20 21 22 23 24 26 ,26 Mean Number of Dolphins Killed per Set FIGURE ~3 Average number of captains with average kill per set for 1989. Most captains killed five or fewer dolphins per set.
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From page 60... ...
is greater than for the other stocks or species (spotted dolphins, 400; eastern spinner dolphins, 160; and whitebelly spinner dolphins, 180) , and their more active behavior in the net makes them more likely to become trapped or tangled (IA11C, 1989c)
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From page 61... ...
A spatial stratification scheme based on one-degree areas is used to increase the precision of the estimates. Flag of Vessel Early estimates of mortality (Allen and Goldsmith, 1981, 1982; Hammond and Tsai, 1983; Hammond, 1984; Hammond and Hall, 1985)
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From page 62... ...
Duration of Set Longer sets have higher mortality values than shorter ones. Typically, the backdown procedure lasts about 15 minutes; however, sets with backdown lasting 40 minutes or more have mortality rates 10 times higher than sets with backdown lasting 20 minutes or less (IA11C, 1989c)
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From page 63... ...
Total U.S. Seiners All Seiners All Dolphin Dolphin Year >400 Tons Seiners >400 Tons Seiners Mortality Mortality 1974 138,901 161,983 107,854 118,475 103,000 175,000 1975 158,567 180,470 122,716 132,848 110,000-194,000 1976 173,734 194,469 124,255 133,361 108,000 128,000 1977 177,421 197,052 122,405 130,221 22,000 51,000 1978 176,054 199,220 113,050 122,559 12,000 31,000 1979 178,765 202,407 113,218 122,941 21,426 17,938 1980 181,991 203,481 117,147 124,614 31,970 15,305 1981 186,219 203,794 113,159 120,216 35,089 18,780 1982 170,786 184,893 109,100 114,840 29,104 23,267 1983 139,594 153,384 81,017 86,922 13,493 8,513 1984 114,871 125,036 48,777 52,700 40,712 17,732 1985 130,146 140,254 50,872 54,494 58,847 19,205 1986 125,740 135,065 44,664 47,645 133,174 20,692 1987 149,909 158,429 43,965 46,245 99,187 13,992 1988 154,399 163,258 46,531 49,026 78,927 19,712 1989 138,349 147,272 33,881 36,236 96,979 12,643 1990 140,910 149,279 33,220 35,412 52,531 5,083 aData from IA1TC; except total mortality data for 1974-1978; in those cases, the low range is extrapolated from a figure produced by a workshop and reported by IA¢C (1989c)
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From page 64... ...
. Sightings made from tuna vessels recorded by IATTC and NMFS observers placed on board to monitor the incidental mortality of dolphins in fishing operations.
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From page 65... ...
Inns Is the species most frequently set on: in the 1984-1988 period, 83.9% of dolphin sets were directed at spotted dolphins, either in pure herds or mLxed with spinner dolphins. Abundance estimates for the northern stock for 1986~1990, based on RVD, average 1,514,800 animals,
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From page 66... ...
Trends for the whitebelly spinners also indicate a stable situation in recent years, having a possible decline in the late 1970s. In 1986-1990,21.9% of dolphin sets involved eastern spinners and 16.5% involved whitebelly spinners; in the great majority of these sets the spinners were in mixed herds with spotted dolphins.
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From page 67... ...
SIGNIIICANCE OF THE ESTIMATES OF MORTALllY AND ABUNDANCE For a complete assessment of the significance of estimated mortality and abundance to dolphin populations, a better knowledge of recruitment rates and dolphin migration is needed, as well as better stock identification of individuals. At present, the eastern spinner dolphin (Stenella longirostris orientalist is the subject of current concern and currently being proposed for depleted status under the MMPA.
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From page 68... ...
. 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 FIGURE ~6 Relative abundance estimates for the northern offshore spotted dolphin.
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From page 69... ...
OVERVIEW: THE PAST, PRESENT, AND FUTURE H;stoncal Summary Clearly, kill rates during the early years of the fishery were significantly reducing the populations of the most commonly caught dolphins. By the early 1970s, when the strictures of the M~PA were being felt, a decline in overall kill rates began under the guidance of NMFS, with the cooperation of the industry's fishery-research arm, the Porpoise Rescue Foundation.
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From page 70... ...
Recent estimates of dolphin abundances in the ETP vary; the best although still impreciseseems to be that of Gerrodette and Wade (in press, b) and is about 8,000,000 animals, with eastern spinners estimated at 600,000.
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From page 71... ...
The committee did not analyze different fishery-management options to conserve dolphin populations, but notes that if better information is obtained on the timing and distribution of dolphin reproduction in the ETP, time and area closures might be worth consideration. In addition, the committee notes that a complete ban on dolphin fishing or the purchase of tuna caught on dolphins is not required to ensure the survival and even the increase of dolphin populations.
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