ABSTRACT
The queen conch, Strombus gigas, is a gastropod of commercial importance in the Caribbean. Population studies are based on size frequency analysis, using either length or weight parameters for the whole live organism. This contribution used mark-recapture data to estimate the von Bertalanffy equation parameters and population number variation within a non harvest population from a protected area, to clarify the biometric parameters that better suit for the whole population, or for the juvenile and adult fractions. Conchs from Xel- Ha Park were monthly sampled from November 2001 to August 2005. Every conch found was measured and marked with a numbered tag that identified month and locality; and monthly abundance was estimated with Jolly’s method. Length, lip thickness and weight increments were used to estimate the von Bertalanffy growth equation parameters with Appeldoorn’s subroutine of FISAT program. The population number varied through the study, with a minimum of 49 in April 2003 and maximum of 9 848 during June 2005. Conchs make only temporary use of Xel-Ha cove. Shell length gave the best fit for the juvenile fraction: L∞=251, K=0.3, C=0.8 Wp=0.3; and lip thickness for adults: L∞=47.78, K=0.17, C=0.1, Wp=0.86, while, the whole population was bet- ter represented by weight: L∞=3850, K=0.36, C=0.8, Wp=0.3. A maximum age of 19 years was estimated from the population. Natural mortality was 0.49/year for juveniles and 0.29/year for adults. There were two pulses of recruitment: fall-winter and summer. It is concluded that population studies from length frequency data, should be analyzed independently in two groups, shell for the juvenile fraction and lip thickness for the adult fraction, or if it is not possible to analyze the population fractions separately, weight should be used to avoid miss calculation of the age structure.
El caracol reina, Strombus gigas, es un gasterópodo de importancia comercial en el Caribe. Los estudios poblacionales se basan en el análisis de las fluctuaciones de las frecuencias de tallas, usando ya sea la longitud de la concha o el peso total de los organismos vivos. Este aporte usa datos de marcaje recaptura para estimar los parámetros de la ecuación de von Bertalanffy y variaciones en el número de la población, para una población sin captura dentro de un área protegida, para aclarar que parámetros se adaptan mejor para estudios de la población entera o para las fracciones de juveniles y adultos por separado. Se muestrearon mensualmente, caracoles del parque Xel-Ha, de Noviembre 2001 a Agosto 2005. Todas las conchas encontradas se se midieron y marcaron con una etiqueta numerada que identificó la fecha de muestréo y la localidad. Las variaciones en abundancia mensual se estimó con el método de Jolly. Se emplearon la longitud de la concha, el grosor de labio y el peso vivo de los animales para determinar los parámeros de la ecuación de crecimiento de von Bertalanffy, usando la subrutina de Appeldoorn del programa FISAT. La población varía a lo largo del estudio, con un mínimo de 49 organismos en abril 2003 y un máximo de 9 848 durante junio 2005. Los caracoles solo hacen uso temporal de la caleta de Xel-Ha. El mejor ajuste para juveniles se obtuvo con la longitud de la concha: L∞=251, K=0.3, C=0.8 Wp=0.3: mientras que para adultos se obtuvo con el grosor del labio: L∞=47.78, K=0.17, C=0.1, Wp=0.86; y la población total se ve mejor representada por el peso: L∞=3850, K=0.36, C=0.8, Wp=0.3. Se estimó una edad máxima para el total de la población de 19 años. Con mortalidad natural de 0.49 año-1 para juveniles y 0.29 año-1 para adultos. Hubieron dos pulsos de reclutamiento: otoño-invierno y verano. Se concluye que los estudios poblacionales realizados a partir de estructura de tallas, deben de analizar por separado dos grupos o fracciones de la población, usando el peso para juveniles y grosor del labio para adultos, de no ser posible analizar la población por separado, debe de emplearse el peso para evitar una subestimación en la estructura de edad.
Subject(s)
Animals , Gastropoda/classification , Conservation of Natural Resources , Gastropoda/anatomy & histology , Gastropoda/growth & development , Mexico , Population Density , Population Dynamics , SeasonsABSTRACT
The need to propose recommendations for the management of over 80 species of bivalves and gastropod mollusks exploited commercially in Mexico, led to look for trends on the population and reproductive biology in relation to prevailing environmental conditions of the habitats where they are exploited. The reproductive cycle, growth parameters for the von Bertalanffy equation, mortality and recruitment of 18 populations of 14 species from 13 localities are compared and related to ambient temperature, precipitation, evaporation, geomorphology, tides, and water salinity and temperature. Localities were classified as influenced by landmasses or with marine influence, with a desert or tropical humid climate. With restricted or continuous communication to oceanic waters and with or without freshwater runoff. The reproductive cycles were classified in four groups in relation to intensity and duration of the spawning season: 1) limited to one annual spawning, 2) two or more defined spawning periods, 3) two or more extended spawning periods, and 4) continuous low intensity spawning. And three groups in relation to gonad recovery, and duration and intensity of gametogenesis: 1) post spawn and rest stages absent or restricted, 2) fast gametogenesis and a clear mature stage. and 3) extended gametogenesis and limited maturity stages. The population parameters were classified in relation to age structure and number of cohorts, intensity and duration of recruitment, and growth rates as expressed by infinity and K. In relation to their life cycle four types were found: 1) population represented by only one cohort, at least during part of the year, 2) with two or more cohorts at any time, 3) longevity under five years, and 4) longevity over live years. In relation to recruitment: 1) one recruitment period restricted to a short season, 2) two or more recruitment periods, 3) constant recruitment with one or more peaks, and 4) constant recruitment without periods of dominance.