Assessing uncertainty in growth and mortality of severely threatened blue land crabs (Cardisoma guanhumi Latreille, 1825), using bootstrapped mark-recapture and length-frequency analysis
Year: 2019
Type: preprint
Abstract: Tagging and length-frequency analysis (LFA) are common methods to study body growth and mortality in populations of fish and invertebrates, but little has been done to assess and compare the inherent uncertainty of these methods. This study applies a new bootstrap-based approach (fishboot R package) to a case study of blue land crabs Cardisoma guanhumi, which are widely harvested and consumed in Brazil. Crabs were sampled at upper mangrove fringes of Itamaraca Island over 12 months, from April 2015. A total of 1,078 individuals were captured and measured. Among these, 291 individuals were marked with PIT tags. 130 size increments were obtained. Carapace widths varied from 20.9 to 70.0 mm (mean: 43.4 mm). Tagging and LFA produced similar median parameter estimates, but tagging produced significantly (p < 0.0001) narrower 95% CIs. Estimates for K from tagging were 3 times more precise than from LFA, estimates for Linf were 2.2 times more precise. Both methods indicate very slow growth and Linf far above Lmax. Tagging-based estimates were K = 0.12 y-1 (95% CI: 0.024 to 0.26 y-1), Linf = 118 mm (95% CI: 81 to 363 mm), Phi' = 1.23 log10(cm y-1) (95% CI: 0.86 to 1.36 log10(cm y-1)). Seasonality in growth was significant (p = 0.006, 95% CI for C: 0.15 to 0.93). A two-step non-parametric bootstrap was used to assess Z and to investigate the log-linear relationship (slope = 1) between Z and input growth parameters, with implications for previous meta-analyses. Total mortality was Z = 2.18 y-1 (95% CI = 1.7 to 4.5 y-1). Age of captured individuals was 1.5 years (20.9 mm) to 7.0 years (70.0 mm). Recruitment was continuous, with an increase in the dry season. Slow growth and a very high Z/K ratio for this population confirm the need for protective measures. Implications for length-based studies of growth and mortality are discussed.
Source: arXiv (Cornell University)
Authors: Ralf Schwamborn, Denise F. Moraes-Costa
Language: English
Subfield: Ecology
Field: Environmental Science
Domain: Physical Sciences
SDG: Life below water
Open Access status: green
OpenAlex ID: W2973169250