Sailfish Growth Study

“A growth model for Indo-Pacific sailfish, Istiophorus platypterus, in the eastern tropical Pacific Ocean using mixture distributions from length frequency data to statistically discern age groups”

 Fitchett, M.D. and N.M. Ehrhardt

 Abstract:

The Indo-Pacific sailfish, Istiophorus platypterus, has been subjected to heavy exploitation throughout the eastern tropical Pacific as non-target catch in industrial high-seas longline and purse seine fisheries in additional directed fisheries in localized artisanal longline and drift net fisheries. In order to accurately assess population processes and mortality by discernable natural and fishery sources, a growth model is imperative. At present, reliable growth models for the species are absent in the region. In this study, spatially explicit observed sailfish length data series (n=9321 measurements eye-forklength in cm) from widespread purse seine fisheries in the eastern tropical Pacific from 1991 to 2007 are used to develop a growth model for sailfish. Methods include fitting mixture distributions representative of age groups partitioned by singular statistical distributions among length-frequency data in the R Statistical Computing environment (R Development Core Team 2010). A simulator is developed to utilize outputs from a series of fitted mixture distributions to recreate length at age by a series of Monte Carlo simulations for each simulated age class. A series of simulated von Bertalanffy growth functions are fitted as a result to estimate much needed growth parameters for the species. Resulting fits yield a von Bertlanffy growth function with asymptotic growth maxima for sailfish to be 207.05 cm eye forklength, with intrinsic Brody growth rate coefficient of 0.59, and a fitted age at size 0 to be -0.04 years. Combined simulated length at age distributions statistically fit to the original length frequency data series for which they are estimated from (Kolmogorov–Smirnov, p > 0.5) thus verifying the validity of the simulation procedure.

Why study growth?

 Understanding sailfish growth is paramount in assessing the population dynamics of the species. Mortality estimations and migratory patterns are contingent on age and size of the animals. In order to assess the age composition, animals need to be sampled and assessed by observations of length and weight, thus rendering the immediate need to link age with the size of the fish.

Von Bertalanffy Growth Model for Sailfish in Eastern Pacific, Fitchett and Ehrhardt (2012)

Age Composition of Sailfish Caught in Eastern Pacific Purse Seine fleet, Fitchett and Ehrhardt (2012)

Frequency of sailfish lengths from Cerdenares-Ladrón De Guevara et al. (2011, Table 1) in green compared with frequencies extracted from purse seine length-frequency database from the same Tehauntepec, MX region.

Histogram of length-frequency data used in the region wide growth analyses for sailfish in the eastern Pacific with a von Bertalanffy growth trajectory slicing through the distribution. Dashed lines denote 95% confidence intervals of size at age.

Parameters for global sailfish von Bertalanffy growth functions (t0, k, LINF ) in peer-reviewed literature, sex, region, authors, measurement type, and length converted to eyeforklength (EFL) by lower-jaw forklength (LJFL) conversion functions by Prince (1986).

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