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Normal (fixed spread) retention curve Deviance residuals 
Normal (proportional spread) retention curve 
Log-normal retention curve 
Deviance residuals 
Deviance residuals 
Figure 3 
Gillnet selectivity curves and residuals estimated for blacktip sharks (Carcharhinus limbatus ) in the Gulf of Mexico 
Shark Pupping and Nursery (GULFSPAN) survey in northwest Florida calculated from the normal (fixed spread), 
normal (proportional spread), lognormal, and gamma distributions. The plots on the left are the estimated gillnet 
selectivity curves with relative retention probability on they axis. Increasing height of the curves indicates increas- 
ing mesh sizes. The plots on the right show the residuals of the models and mesh size on the y axis increases from 
bottom to top. Filled circles represent positive residuals and open circles represent negative residuals. The area of 
the circle is proportional to the square of the residual. 
there may have been a year effect that we were unable 
to account for, this is unlikely because of the nature 
of the survey and the species studied. Generally, year- 
to-year variability in recruitment is lower in sharks 
than in teleosts because of the production of large, 
well-developed young and low natural mortality (Smith 
et al., 1998; Walker, 1998). The GULFSPAN survey 
primarily targets juvenile sharks in nursery areas, 
and the majority of the blacktip sharks captured were 
juveniles. Therefore it is probable that interannual 
size variability was low in the survey area for black- 
tip sharks. Although this is an important factor that 
could be applied to other selectivity studies with more 
robust sample sizes, current stock assessment models 
