Byrd et at: Effects of commercial fishing regulations on stranding rates of bottlenose dolphin ( Tursiops truncatus ) 
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Figure 2 
Spiny dogfish ( Squalus acanthias) landings in metric tons (denoted by bars) and numbers of 
bottlenose dolphin ( Tursiops truncatus) strandings (denoted by a line) classified as HI-FI (i.e. , 
having evidence of fishery interaction) in North Carolina from November 1997 through April 
2005. Shaded areas represent winter months (November-April). Asterisks (*) represent months 
when minimal landings occurred. No bars or asterisks represent months with no landings. 
Monthly strandings in winter decreased between time period (TP) 1 and TP2, delineated by 
a dashed vertical line. 
used to help detect unusual mortality events (UME) for 
overall strandings by the Marine Mammal Health and 
Stranding Response Program (Wilkinson, 1996) and was 
termed the “UME threshold method.” It was calculated 
as the mean number of strandings (in this case HI-FI) 
per month plus two standard deviations (SD). Stranding 
thresholds were calculated for each bottlenose dolphin 
MU in NC. For the NC winter mixed MU, data collected 
in TP2 were used. The stranding threshold was then 
compared to monthly HI-FI strandings during TP1 and 
TP2 to determine whether it serves as an adequate indi- 
cator of relative bycatch levels. For the summer northern 
NC MU, the stranding threshold also was calculated 
with data collected in TP2. For the summer southern 
NC MU, the stranding threshold was calculated with 
data from TP1 and TP2 because estimated bycatch levels 
never exceeded PBR levels in TP1. 
The second method that was investigated to establish 
stranding threshold levels was based on the regression 
analysis of seasonal HI-FI strandings and estimated 
bycatch. This method used the maximum likelihood es- 
timates to calculate the predicted values of bycatch and 
the 68% confidence intervals (CIs) and 95% CIs. The Cl 
values of predicted bycatch rates were then evaluated to 
determine if they would be appropriate for identifying 
periods of elevated bycatch. 
Results 
From November 1997 through April 2005, NC gillnetters 
landed 6310 t (metric tons) of spiny dogfish. Landings 
occurred almost entirely in winter (November-April), 
and less than 0.1% occurred in other months (Fig. 2). 
More than 96% of all landings occurred before Novem- 
ber 2000 (TP1). During winter, mean landings were 
2020 t (SD = 561) per fishing season during TP1 and 49 
t (SD = 104) during TP2. After November 2000, 96% of 
these landings occurred during the 2003-04 fishing year. 
During the same time period (November 1997 through 
April 2005), 439 bottlenose dolphin strandings met the 
criteria for inclusion in this study. Overall, more strand- 
ings occurred during winter than summer in each HI 
category (Table 1). For all years, HI-CBD strandings 
comprised 60% of winter (range: 45—69%) and 52% of 
summer (range: 25-65%) totals (Hl-yes, Hl-no, and 
HI-CBD). HI-FI strandings comprised 22% of winter 
(range: 11-35%) and 21% of summer (range: 11-33%) 
totals for all years. However, of strandings for which 
it was possible to determine whether an interaction 
occurred (HI-FI, Hi-other, and Hl-no), HI-FI strand- 
ings comprised 56% (range: 27-75%) of winter and 44% 
(range: 20-57%) of summer totals for all years. 
Rates of HI-FI strandings had a similar pattern to 
that of bycatch estimates and effort in the spiny dogfish 
fishery. There was a significant positive relationship be- 
tween the number of HI-FI strandings and the bycatch 
estimate per season (r 2 = 0.79, P=0.0003). Additional- 
ly, the mean number of winter HI-FI strandings per 
month was significantly greater during TP1 than TP2 
(P=0.001) (Table 2). There was no significant difference 
in winter Hl-no or HI-CBD strandings between TP1 and 
TP2. HI-FI strandings showed a monthly periodicity 
similar to that for fishing effort during TP1 (Fig. 2); 
four to six animals were recovered per month during 
the height of the fishery, compared to generally two or 
