Philippine Journal of Science 150 (3): 1051-1060, June 2021 ISSN 0031 - 7683 Date Received: 21 Dec 2020 Population Dynamics of the Comb Pen Shell Atrina pectinata (Linnaeus, 1767) (Mollusca, Bivalvia: Pinnidae) Collected by Diving from Shallow Areas of the Southwest Visayan Sea, Northeastern Panay Island, Philippines Annabelle del Norte-Campos*, Switzel S. Lapara, and Kris Angeli S. Sanchez Marine Biology Lab, Division of Biological Sciences College of Arts and Sciences, University of Philippines Visayas Miagao, Iloilo, Philippines With the objective of determining the population dynamics of the comb pen shell (Atrina pectinata) (Mollusca, Bivalvia: Pinnidae) collected by divers in the southwest Visayan Sea, length-frequency data recorded between June 2018 and May 2019 were analyzed using the FiSAT software. von Bertalanffy growth parameters for the species were SL = 40.69 cm and K = 0.78 yr–1 the range of values for other fast-growing tropical bivalves. The estimated mean growth rate of 0.15 ± 0.05 cm d–1 likewise supports a fast growth rate and a short lifespan (< 1 yr) of the population. The recruitment pattern shows one major and minor pulse that coincides with the northeast monsoon and just prior to the southwest monsoon, respectively. Using ELEFAN II, total mortality (Z) of 2.79 yr–1 was estimated. From an averaged M/K value from the related bivalve literature (1.51) multiplied by the species’ own K, natural mortality (M) of 1.18 yr–1 was estimated, which when subtracted from the Z, gave fishing mortality (F) of 1.61 yr-1 and an exploitation rate E of 0.58, considered overexploited. Recommended management measures for more sustainable utilization of the species include non-collection of individuals smaller than the cited minimum size at sexual maturity [20 cm shell length (SL)], and a ban on the collection at least during the major spawning season deemed to be, based on the growth rate between October and December each year. Marketing of the non-adductor portion of the viscera, normally just discarded, is also recommended to increase the income of the fishers. Keywords: Atrina pectinata, exploitation, growth, mortality, population dynamics, recruitment INTRODUCTION – as reported by Yang et al. (2015) in Ulleungdo Island, Korea having lifespans of up to 7.5 yr. A characteristic of The comb pen shell Atrina pectinata (Figure 1), locally Family Pinnidae (pen shells) is that they are found partly known as tarab, is widely-distributed in the Indo-west embedded in heterogeneous soft bottoms, with the narrow Pacific. It ranges from the region’s tropical to subtropical anterior tip of their shell directed downwards. The outer areas (Poutiers 1998). It is tolerant to a wide range of surface color of the valves of A. pectinata is a translucent temperature (1–39 ºC) with those living in colder latitudes olivaceous tan, often tinged with darker purplish-brown or gray towards the umbones (Poutiers 1998). A. pectinata *Corresponding Author: [email protected] has 15–30 radial ribs, while the internal nacreous layer is 1051 Philippine Journal of Science del Norte-Campos et al.: Population Dynamics Vol. 150 No. 3, June 2021 of A. pectinata in Visayan Sea, Philippines Figure 1. The comb pen shell Atrina pectinata (Linnaeus, 1767) here shown with a measuring tape oriented along the anterior-posterior (SL) axis used in this study. undivided and the posterior adductor scar is completely In the study of Diocton and Adalla (2019); however, a enclosed within the nacreous area. The species can easily very small number (N = 397) of size-biased individuals be distinguished from the related co-occurring species, for the one-year study was analyzed. Aside from the poor the bicolor pen shell Pinna bicolor, which has a median representation of the natural population in their study, the radial groove that is absent in Atrina. estimation of natural mortality M was also incorrectly based on Pauly’s empirical formula, which was derived A. pectinata was ranked number one in an inventory only from fish stocks (Pauly 1980). A more correct method on commercially-important mollusk species conducted would have been to take M and K values from related in Panay Island, Philippines (del Norte-Campos et al. bivalve literature, and the mean M/K of these bivalves 2019). This inventory used a set of criteria that includes multiplied by the K of the species in question to get M, a commercial value, catch rates, sources of threats, simple arithmetic procedure (Pauly, pers.com.). frequency in markets and source sites, and literature available. It is the only mollusk species in Panay whose To ensure the long-term sustainability of the species, there adductor muscles are sold in a nearby processing plant in is a need to understand its population dynamics to assess San Dionisio, Iloilo for export to Taiwan and, therefore, exploitation that is necessary in formulating science- it rated highest in terms of commercial importance the based management guidelines. Thus, this paper aims to criterion given the highest weight in the inventory. study the population dynamics of A. pectinata from San Dionisio, Iloilo. Specifically, it aims to determine growth In Panay, Western Visayas, Philippines, A. pectinata is and mortality parameters, the recruitment pattern, and the being collected by an unregulated small diving fishery exploitation rate. – rather wasteful in that only the adductor muscles are marketed and the rest of the viscera are discarded. An annual catch of 22.1 MT yr-1 was estimated, making it the number one species (86.3%) of the four exploited MATERIALS AND METHODS species (including Placuna ephippium, Anadara globosa, and Laevistromus turturella) in the diving fishery of the Study Area and Sample Collection kg–1, this annual catch has a corresponding value of PHP The study area is located in the southwest Visayan Sea, 4.4 M yr-1. At present, 25 divers operate in the area; thus, western Visayas, Philippines (Figure 2), where a diving the annual value of the catch corresponds to an income fishery exists and catches are landed in San Dionisio, of about PHP 176T diver-1 yr-1. Most of the works on Iloilo. Diving is conducted in shallow subtidal areas, the species either cover genetics (Dong et al. 2014) and approximately 3–7 m deep with the bottom substrate reproductive biology (Yang et al. 2016). The only work ranging from coarse to fine sediment. The catches of published on its population biology in the tropics is that of a fixed number of divers (six) based in San Dionisio, Diocton and Adalla (2019) in Maqueda Bay, Philippines. Iloilo (Figure 2) were sampled every week from June 1052 Philippine Journal of Science del Norte-Campos et al.: Population Dynamics Vol. 150 No. 3, June 2021 of A. pectinata in Visayan Sea, Philippines Figure 2. Location of the study area in San Dionisio, Iloilo, Western Visayas, Philippines where pen shells were collected and measured from June 2018 to May 2019 from catches of divers operating in its coastal waters forming the southwest Visayan Sea. 2018 to May 2019, with consideration to include the The value of b was different from 3 (i.e. allometric growth) entire size spectrum. These samples were the ones if the t statistic was greater than the tabulated t value for included in the length-frequency analysis. Given the n – 2 df (Pauly 1984). bulk of the specimens, a separate random but equally sized-representative sample was taken to the lab for the Monthly measurements of SLs (L) of individuals were determination of the species’ length-weight relationship. grouped into 2.5-cm size classes and analyzed using the For this, the SLs of a total of 110 individuals were Bhattacharya (1967) method, which is useful for splitting measured to the nearest 0.1 cm using a measuring tape a composite distribution into separate normal distributions (Figure 1) and total weights (TW) to the nearest 0.01 g when several age groups (cohorts) are contained in the were taken with the use of an electronic balance. same sample. The Bhattacharya routine is incorporated in the FiSAT software (Gayanilo et al. 2005). To compute growth rates (cm day–1) by cohort, increments between Data Analysis modal lengths (mm) derived using the Bhattacharya The length-weight relationship of the species was method were divided with their respective time increments computed using Equation 1: (d). Time intervals between months were uniformly pegged at 30 d inasmuch as measurements during the (1) month were regularly made once weekly. Mean growth rates for each derived cohort were averaged to estimate where TW is measured in g, L is the SL (cm), a is the the annual mean growth rate (cm day–1) for all cohorts. intercept, and b is the slope. The von Bertalanffy growth function (VBGF) parameters To test if the computed b value was significantly different SL (cm) and K (yr–1) were estimated with a fixed starting from 3 (i.e. allometric growth), the value of the t statistic point (Sample 1, L = 22.5 mm) using ELEFAN I. To was computed (Pauly 1984): assess these growth parameters in relation to other bivalve species, the growth performance index (ø’) was computed (2) using the equation (Pauly and Munro 1984): (3) where sd(x) is the standard deviation of the log L (SL) values, sd is the standard deviation of the log W (TW) (y) where K is the growth constant (yr–1) and L is the values, r2 is the correlation coefficient, and n is the number asymptotic length in cm. of individuals used in the computation. 1053 Philippine Journal of Science del Norte-Campos et al.: Population Dynamics Vol. 150 No. 3, June 2021 of A.