Survival of Juvenile Lemon Sharks (Negaprion brevirostris)
at Bimini, Bahamas,
Estimated by Mark Depletion

Samuel Grubergruber@rsmas.miami.edu, Bimini Biological Field Station, University of Miami; Jean de Marignac, Moss Landing Marine Laboratory; John Hoenig, Virginia Institute of Marine Science.

Introduction

If in an unexploited lemon shark population

• 20 yr = lifespan
• 88% = annual survival after the first year
• 12 yr = age of maturity
• 1.5 = female pups per mature female per year

Then survival in first year would equal 39 % (Hoenig & Gruber (1990) Leslie matrix model) if the population is in equilibrium.

Some Questions:

Do we believe this?
It's a theoretical exercise, sensitive to inputs
Is survival variable?
What happens if fewer pups are produced?

Measuring First-Year Survival - Our Field Work Context females pup in Bimini Lagoon in April-May juveniles inhabit nursery areas for several years juveniles have high site fidelity & limited home ranges pups may move from North Sound to Sharkland

Pictures from Our Sampling

Lemon shark caught in a gillnet

Bottom-up view of volunteers removing a lemon shark from the net

Jean de Marignac measuring a lemon shark

Weighing a lemon shark

Field Methods annually from 1995 - 1999 set 3 gillnets at night, up to 6 nights, in North Sound in June. (check nets every 15 min or less) examine, tag & retain catches in holding pen release sharks at end repeat in Sharkland

Analysis

    1) number of sharks present

Analysis

    2) survival of tagged sharks

a) do Leslie analysis on tagged sharks one year later in North Sound

b) estimate emigration of tagged sharks to Sharkland

Results

    Estimated survival rates using

year	observed emigration	computed emigration
1995	.40	.90 *
1996	.62	.62
1997	.36	.36
1998	.60	.60

this is not credible because the Leslie model did not fit well

Conclusions

• can estimate survival in first year of life
• survival estimates range from 36 to 62 % survival
• survival inversely related to pup density
• we begin to get an idea of pup production capacity in North Sound: approximately 28 pups can be supported
• tentative model of pup survival in relation to stock size

If extrapolate back to 0, have a model for effects of fishing (extrapolation is unreliable but it's the best we can do)

If extrapolate back to 0, have a model for effects of fishing that can be applied to other species