Length – Weight and Length

Philippine Journal of Science 146 (1): 85-94, March 2017 ISSN 0031 - 7683 Date Received: ?? Feb 20??

Length – Weight and Length – Length Relationships, Condition Factor, Sex Ratio and Gonadosomatic Index of the Ariid Catfishes Arius dispar and Arius manillensis (Siluriformes: Ariidae) in Laguna de Bay, Philippines

Brian S. Santos1, Reynand Jay C. Canoy1, 2, Jazzlyn M. Tango-Imperial1,3, and Jonas P. Quilang1*

1Institute of Biology, College of Science, University of the Philippines, 1101 Diliman, Quezon City, Philippines 2Institute of Human Genetics, National Institutes of Health, University of the Philippines Manila, 625 Pedro Gil St, Ermita, Manila, 1000 Philippines 3Department of Biology, College of Science, Bicol University, 4500 Legazpi City, Philippines

The ariid catfishes Arius dispar and Arius manillensis are commercially important in the Philippines and have been overexploited in the past. This study describes for the first time the length-weight and length-length relationships, condition factor, sex ratio, and gonadosomatic index of the two species. A total of 1,698 A. dispar and 874 A. manillensis were collected from Laguna de Bay over the period of 12 months to assess the aforementioned parameters. For both species, the sex ratio significantly differed from equality, the length-length relationships were highly significant and the coefficients of determination (r2) were all greater than 0.96. Length frequency analysis indicates overfishing for both species. The average monthly gonadosomatic index (GSI) ranged from 0.04 to 0.15 in A. dispar males and from 0.23 to 2.99 in females. The average monthly GSI ranged from 0.04 to 0.49 in A. manillensis males and from 0.28 to 4.02 in females. For females of each of the two species, the GSI had two peaks: one from February to May (dry months) and the other from July to September (wet months). These peaks might correspond to the spawning runs of these two species. This study provides baseline information which can be used for the management and conservation of these economically important fishery resources.

Key words: Arius, comparative growth, gonadosomatic index, fishery management, reproductive period

INTRODUCTION was described and named by Herre in 1926 from the specimens he obtained from Laguna de Bay, Pasig River, The ariid catfishes Arius dispar Herre, 1926 and A. and Quiapo Market in Manila. Arius manillensis is an manillensis Valenciennes, 1840 are both commercially endemic species in the Philippines. Both species, locally important in the Philippines. A. dispar is a native species known as kanduli, are found in Laguna de Bay, the largest in the Philippines and is found also in Taiwan and possibly lake in the Philippines, with an area of about 90,000 ha northern Borneo (Kailola 1999). The species A. dispar and an average depth of 2.8 m. The lake is located on the Philippine island of Luzon and lies in about 14°0’0” *Corresponding author: [email protected] N latitude and 121°5’59” E longitude. In addition to A.

85 Philippine Journal of Science Quilang et al.: Length-Weight and Vol. 146 No. 1, March 2017 Gonadosomatic Index of Arius species dispar and A. manillensis, three other species of kanduli This study was aimed at determining the length-weight have been reported in Laguna de Bay, namely, Arius relationships, length-length relationships, condition factor, goniaspis, A. thalassinus, and Hemipimelodus manillensis sex ratio, and gonadosomatic index of A. dispar and A. (Herre 1926; Aldaba 1931). Mane (1929) and Aldaba manillensis. This study provides baseline information (1931) reported that commercial catches of kanduli which can be useful to fishery managers and policy consisted mainly of A. manillensis and that the other makers for the sound management and conservation of the species were very rare. Only A. dispar and A. manillensis two species. The mathematical description of length and were found in this study and in the study by Vallejo weight can be used to convert one into the other (Tesch (1985), Santos and Quilang (2012), and Yu and Quilang 1968). When one is in the field, it is easier, faster, and more (2015). Kanduli used to be the most abundant among all accurate to measure length than it is to measure weight. the fishes found in Laguna de Bay (Mane 1929; Aldaba Further, length-weight relationships may be used to infer 1931), but beginning the early 1930s, depletion of the differences between stocks or populations or differences fishery resources for kanduli had been noted (Villadolid between the stages in the life history of a species (Le Cren 1933, 1934). Kanduli have become very rare in the 1951). Length-length relationships for fork length, total 1960s (Delmendo & Bustillo 1968) and 1970s (Mercene length and standard length are also considered very useful 1978). The populations of kanduli have recovered since for comparative growth studies (Moutopoulos & Stergiou then so that in a survey conducted in 1995-1996, among 2002; Hossain et al. 2006; Dadzie et al. 2008). Differences all the fishery resources in the lake, the average annual in condition factors can be used as measure of the fatness, fish catch for kanduli (reported by the authors as A. well-being, or the suitability of the environment to a manillensis) was the third highest at 516.9 MT (Palma species (Le Cren 1951). et al. 2002). The yield (catch) to biomass ratio, however, was 4.4, indicating a state of overexploitation (Palma et al. 2002). Despite the economic importance of these fishery resources, very few studies have been conducted MATERIALS AND METHODS on them. Mane (1929) conducted a preliminary study on A total of 1,698 A. dispar (907 males and 791 females) the feeding, spawning, breeding, and migratory habits of and 874 A. manillensis (230 males and 644 females) were A. manillensis. Stomach content analysis revealed that collected monthly from Laguna de Bay beginning October mature fish is largely carnivorous, consuming mainly 2009 until September 2010. The specimens were obtained fish, snails, shrimps, crabs, earthworms, and insects, and from local fishermen who caught the fish using gill nets to a lesser extent, vegetable matter (Mane 1929). About and fish traps. The specimens were transported on ice; 40% of the diet of young fish consisted of algae and measurements and dissection were immediately done upon plant materials such as duckweed; the remaining portion arrival in the laboratory. The specimens were identified consisted of animals such as crustaceans, mollusks, based on species descriptions and identification keys earthworms, and fish (Mane 1929). Mane (1929) also in Conlu (1986), Herre (1926), Kailola (1999), Ferraris observed that kanduli are initially pelagic, but as the (2007), and Marceniuk & Menezes (2007). The most breeding season approaches, mature fish gather in schools striking difference between A. manillensis and A. dispar and migrate to the deeper portions of the lake where they is their palatal tooth patch morphology. A. manillensis spawn. After spawning, mature spent females leave the has palatal tooth patches that are more oval than elongate, breeding ground and the fingerlings that develop then while A. dispar has small and widely separated tooth move to the shore. Delmendo (1968) also studied the patches (Kailola 1999). Aside from these, the two species food and feeding habits of Arius sp. (exact species not are similar morphologically. indicated) in Laguna de Bay. In addition to food items such as shrimp, snail, fish, chironomid larvae, insects, Standard length (SL), total length (TL), and fork length and vegetable materials, Delmendo (1968) found sand and (FL) were measured to the nearest 0.1 cm. Body weight silt particles in the stomach of kanduli, which indicated (BW) was taken to the nearest 0.1 g and gonad weight that they are bottom feeders. To date, there is no study (GW) to the nearest 0.0001 g. The sex of each fish was yet specifically on the biology of A. dispar. All the other determined by examining the gonads. Standard length studies conducted on kanduli (Arius spp.) were on the (SL; measured in cm) and BW (in g) were log-transformed possible causes of depletion and suggested regulatory and the length-weight relationship was determined using measures for proper management (Villadolid 1933, 1934), the allometric equation BW = a ×SLb, where a and b are fish catch landing statistics and on the types of fishing regression parameters. Linear regression of Log10BW on gears used to catch the species (Aldaba 1931; Delmendo Log10SL was done to determine length-weight relationship & Bustillo 1968; Mercene 1978, 1987). separately for males and females and both sexes combined for each monthly collection and for the pooled samples.

86 Philippine Journal of Science Quilang et al.: Length-Weight and Vol. 146 No. 1, March 2017 Gonadosomatic Index of Arius species

Following Zar (2010), comparison of regression parameter RESULTS b between males and females for each species was done using t-test and comparison between months was done Of the 1,698 Arius dispar specimens, 907 (53.4%) were using Analysis of Covariance (ANCOVA). Length- males and 791(46.6%) were females; the sex ratio was length relationships of TL vs SL, SL vs FL, and FL vs 1.15 male: 1 female, which differed significantly from 2 TL were also determined using linear regression analysis. equality ( χ = 7.925, P = 0.005). On the other hand, The Fulton condition factor (K) was computed for each of the 874 A. manillensis specimens, 230 (26.3%) were 3 specimen using the equation: K = (BW∕SL ) × 100. The males and 644 were females (73.7%); the sex ratio of 2.8 gonadosomatic index (GSI) was computed using the females:1 male also differed significantly from equality equation: GSI (%) = GW ×100 ∕ (BW– GW). All statistical 2 analyses were performed using SPSS software (SPSS, ( χ = 196.105, P = 1.48×10-44). The length frequency Chicago, IL, USA). The length-frequency analysis wizard distributions for both species are shown in Figures 1 on FishBase (Froese 2004) was used to determine the and 2. For both species, the graphs are approximately extent of overexploitation of the two species. bell-shaped. For male specimens of A. dispar, 20%

Figure 1. Length frequency distribution of Arius dispar from Laguna de Bay, Philippines. No. of specimens: males = 907; females = 791.

Figure 2. Length frequency distribution of Arius manillensis from Laguna de Bay, Philippines. No. of specimens: males = 230; females = 644.

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have standard lengths (SL) between 10 and 16 cm, 68% classes below the Lopt. The percentage of specimens below between 17 and 20 cm, and 12% between 21 and 30 cm. Lopt-10% was 62.9% for A. manillensis and 75.7% for A.

For female specimens, 12% have SL between 11 and 16 dispar. The percentage of specimens above Lopt+10% was cm, 68% between 17 and 20 cm, and 20% between 21 and 2.7% for A. manillensis and 1.9% for A. dispar. 34 cm. For the male specimens of A. manillensis, 11% have SL between 13 and 16 cm, 71% between 17 and 20 The average monthly Fulton’s condition factor (K) cm, and 18% between 21 and 28 cm. For females, 5% have ranged from 1.31 to 1.55 in A. dispar males and from SL between 13 and 16 cm, 59% between 17 and 20 cm, 1.32 to 1.60 in females (Figure 3). The mean K (1.47) and 36% between 21 and 36 cm. For A. manillensis, 36 of all the 907 A. dispar males was significantly different cm constitutes a new maximum length. For both species, from the mean K (1.41) of all the 791 females (P value specimens below 10 cm were not included in the study < 0.001). For either of the sexes, there were significant because of the difficulty in determining their sexes and differences in mean K between months (P value < 0.001). identifying their species. For A. manillensis, monthly K ranged from 1.34 to 1.62 for males and from 1.38 to 1.65 in females (Figure 4). The regression parameters and the coefficients of The mean K (1.42) for all the 230 A. manillensis males determination for the length-weight relationships (LWR) was significantly different from the mean K (1.49) of are given in Table 1 for A. dispar and Table 2 for A. all the 644 females (P value < 0.001). There were also manillensis. The b values for male specimens of A. significant differences in mean K between months for dispar varied between 2.7 (October and November) and each of the sexes (P value < 0.001). 3.3 (April). The b values for female specimens varied between 3.0 (November) and 3.5 (April). When all the The average monthly gonadosomatic index (GSI) ranged samples in all the months were pooled, the b values for all from 0.04 to 0.15 in A. dispar males and from 0.23 to 2.99 the specimens, for all males only, and for females only, in females. The average monthly GSI ranged from 0.04 were 3.2, 3.1, and 3.3, respectively, all indicating positive to 0.49 in A. manillensis males and from 0.28 to 4.02 in allometric growth. The slope for males was significantly females. For both species the GSI in females starts to rise different from females (P value < 0.001). ANCOVA in February, reaches its peak in April then falls until June also showed significant difference when slopes were and then peaks again in July until September and then falls compared between months (P < 0.01). On the other from October until January (Figures 5 and 6). hand, the slopes for the male specimens of A. manillensis varied between 2.8 (June) and 3.6 (July). The slopes for female specimens varied between 2.8 (September) and 3.4 (January, July, and August). Pooled samples DISCUSSION for all the months gave b values of 3.1 for males only The two Arius species are always hauled together when and 3.3 for females only and for both sexes combined, fished. They look very similar externally and can only also indicating positive allometric growth. The slope be distinguished from each other by opening the mouth for male specimens of A. manillensis was significantly of the fish and examining the shape of the palatal tooth different from females (P value < 0.01). When males patch. Since the gears used were nonselective and the and females were combined and the regression equation effort exerted in catching the two species were the was computed for each month, ANCOVA showed same, A. dispar can be considered the more abundant significant difference between months (P value < 0.01). species as it outnumbers the endemic A. manillensis The coefficients of determination (r2) for LWR ranged by 2:1. In contrast, Mane (1929) and Aldaba (1931) from 0.85 to 0.98 and from 0.91 to 0.98 for A. dispar reported that the kanduli population in Laguna de Bay males and females, respectively. When both sexes were and in markets near the lake consisted predominantly combined for each month, r2 values ranged from 0.90 to of A. manillensis. 0.98. For A. manillensis, r2 values ranged from 0.87 to 0.995, 0.88 to 0.99, 0.89 to 0.99 for males, females, and The sex ratio for both species differed significantly for both sexes combined, respectively. from the expected 1:1. Quite notably, A. manillensis specimens exhibited a sharp deviation from equality The length-length relationships (LLRs) are given in Table with only a fourth of the sample being males whereas 3 for A. dispar and Table 4 for A. manillensis. The r2 values for A. dispar, almost 47% are males. Although large for both species are greater than 0.96. All LLRs were also female bias in sex ratio has been observed in other highly significant (P value < 0.001). Based on the length- catfish species (Liang et al. 2005), the difference in frequency analysis wizard, the length with optimum yield the sex ratio observed between A. manillensis and A.

(Lopt) was 23.3 cm for A. manillensis and 22.0 cm for A. dispar is remarkable considering the two species are dispar. For both species, the frequency peaks at length very similar in many other aspects.

88 Philippine Journal of Science Quilang et al.: Length-Weight and Vol. 146 No. 1, March 2017 Gonadosomatic Index of Arius species

Table 1. Monthly descriptive statistics and regression parameters of length-weight relationships for males, females, and both sexes combined for Arius dispar from Laguna de Bay, Philippines. M, males; F, females; n, number of individuals; a, intercept; b, slope; CI, confidence interval; r2, coefficient of determination.

Month Standard length (cm) Regression parameters 95% CI Sex N r2 min max a b of b October 2009 M 16 17.0 22.2 0.0347 2.687 2.288−3.086 0.937 F 17 15.6 25.8 0.0107 3.096 2.766−3.427 0.964 M+F 33 15.6 25.8 0.0174 2.925 2.677−3.174 0.949 November 2009 M 64 13.9 24.4 0.0324 2.708 2.457−2.960 0.882 F 55 13.9 21.6 0.0128 3.045 2.854−3.236 0.951 M+F 119 13.9 24.4 0.0192 2.898 2.734−3.063 0.912 December 2009 M 111 10.8 24.5 0.0132 2.995 2.846−3.144 0.936 F 83 10.9 23.2 0.0090 3.135 2.999−3.270 0.963 M+F 194 10.8 24.5 0.0111 3.057 2.955−3.160 0.948 January 2010 M 64 10.3 29.9 0.0096 3.134 3.026−3.242 0.982 F 33 11.2 26.3 0.0080 3.208 3.054−3.362 0.983 M+F 97 10.3 29.9 0.0087 3.173 3.087−3.259 0.983 February 2010 M 180 11.9 29.6 0.0120 3.046 2.935−3.158 0.942 F 98 13.7 24.7 0.0100 3.117 2.943−3.292 0.929 M+F 278 5.2 29.6 0.0114 3.066 2.972−3.161 0.937 March 2010 M 130 15.2 27.9 0.0080 3.194 3.078−3.309 0.959 F 119 15.1 27.2 0.0057 3.317 3.171−3.463 0.946 M+F 249 15.1 27.9 0.0070 3.244 3.154−3.335 0.953 April 2010 M 28 16.4 23.6 0.0058 3.317 2.966−3.669 0.935 F 40 16.9 23.8 0.0039 3.460 3.142−3.777 0.928 M+F 68 16.4 23.8 0.0043 3.419 3.196−3.641 0.934 May 2010 M 60 17.8 28.9 0.0153 2.964 2.639−3.289 0.852 F 55 17.0 32.3 0.0082 3.182 2.932−3.431 0.925 M+F 115 17.0 32.3 0.0095 3.128 2.937−3.320 0.903 June 2010 M 59 14.4 28.4 0.0150 2.972 2.787−3.157 0.948 F 35 14.7 26.4 0.0067 3.244 3.073−3.416 0.978 M+F 94 14.4 28.4 0.0110 3.079 2.948−3.210 0.960 July 2010 M 47 15.5 23.7 0.0087 3.177 2.948−3.406 0.946 F 62 16.1 34.2 0.0093 3.1668 3.007−3.329 0.963 M+F 109 15.5 34.2 0.0080 3.212 3.092−3.332 0.963 August 2010 M 77 13.8 22.2 0.0115 3.080 2.851−3.309 0.905 F 84 16.6 24.5 0.0075 3.237 3.011−3.462 0.909 M+F 161 13.8 24.5 0.0081 3.207 3.051−3.362 0.913 September 2010 M 71 14.6 25.6 0.0142 3.027 2.775−3.279 0.893 F 110 15.5 23.5 0.0102 3.153 2.969−3.337 0.914 M+F 181 14.6 23.5 0.0108 3.129 2.981−3.276 0.907 Overall M 907 10.3 29.9 0.0099 3.119 3.068−3.170 0.942 F 791 10.9 34.2 0.0066 3.273 3.219−3.327 0.948 M+F 1698 10.3 34.2 0.0078 3.208 3.171−3.245 0.944

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Table 2. Monthly descriptive statistics and regression parameters of length-weight relationships for males, females, and both sexes combined for Arius manillensis from Laguna de Bay, Philippines. M, males; F, females; n, number of individuals; a, intercept; b, slope; CI, confidence interval; r2, coefficient of determination. Standard Regression Month Sex n length (cm) parameters 95% CI of b r2 min max a b October 2009 M 5 17.4 23.5 0.0042 3.413 2.744−4.081 0.989 F 11 17.0 25.2 0.0080 3.197 2.728−3.667 0.963 M+F 16 17.0 25.2 0.0067 3.259 2.935−3.583 0.971 November 2009 M 5 15.8 20.8 0.0038 3.456 1.593−5.319 0.921 F 30 16.0 23.4 0.0081 3.197 2.840−3.553 0.923 M+F 35 15.8 23.4 0.0071 3.241 2.931−3.550 0.932 December 2009 M 7 14.6 20.6 0.0056 3.327 3.055−3.598 0.995 F 27 14.2 25.4 0.0052 3.338 3.179−3.497 0.987 M+F 34 14.2 25.4 0.0056 3.310 3.170−3.450 0.986 January 2010 M 11 14.7 26.2 0.0049 3.363 3.022−3.703 0.982 F 18 13.0 29.0 0.0041 3.434 3.264−3.604 0.991 M+F 29 13.0 29.0 0.0043 3.413 3.266−3.559 0.988 February 2010 M 65 13.4 28.0 0.0164 2.946 2.757−3.135 0.939 F 108 14.9 30.4 0.0066 3.268 3.137−3.398 0.959 M+F 173 13.4 30.4 0.0092 3.149 3.040−3.258 0.950 March 2010 M 43 15.5 25.6 0.0059 3.295 3.046−3.545 0.945 F 146 16.1 33.0 0.0071 3.240 3.085−3.395 0.922 M+F 189 15.5 33.0 0.0066 3.262 3.132−3.392 0.929 April 2010 M 7 17.2 20.6 0.0079 3.202 1.956−4.448 0.897 F 42 17.4 26.0 0.0059 3.316 3.044−3.587 0.938 M+F 49 17.2 26.0 0.0054 3.343 3.098−3.588 0.941 May 2010 M 26 17.7 22.6 0.0100 3.099 2.731−3.466 0.926 F 55 17.2 33.1 0.0103 3.108 2.961−3.254 0.972 M+F 81 17.2 33.1 0.0088 3.153 3.013−3.294 0.962 June 2010 M 21 16.5 21.7 0.0249 2.795 2.268−3.322 0.866 F 41 16.2 36.0 0.0077 3.207 3.044−3.371 0.976 M+F 62 16.2 36.0 0.0076 3.210 3.069−3.351 0.972 July 2010 M 12 15.9 21.0 0.0025 3.588 2.979−4.198 0.945 F 53 17.0 28.2 0.0051 3.377 3.032−3.723 0.883 M+F 65 15.9 28.2 0.0032 3.521 3.249−3.793 0.914 August 2010 M 18 17.6 24.4 0.0136 3.036 2.675−3.397 0.952 F 51 16.2 27.0 0.0047 3.391 3.194−3.588 0.961 M+F 69 16.2 27.0 0.0059 3.314 3.146−3.483 0.958 September 2010 M 10 17.1 20.7 0.0155 3.014 2.115−3.912 0.882 F 62 16.7 24.0 0.0267 2.837 2.564−3.110 0.878 M+F 72 16.7 24.0 0.0238 2.875 2.629−3.121 0.886 Overall M 230 13.0 36.0 0.0107 3.095 2.988−3.203 0.934 F 644 13.4 28.0 0.0066 3.269 3.206−3.331 0.942 M+F 874 13.0 36.0 0.0068 3.256 3.202−3.309 0.943

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Table 3. Length-length relationships between total length (TL), Table 4. Length-length relationships between total length (TL), standard length (SL) and fork length (FL) of Arius dispar standard length (SL) and fork length (FL) of Arius from Laguna de Bay, Philippines. manillensis from Laguna de Bay, Philippines. Sex Equation n a b r2 Sex Equation n a b r2 Male TL = a + bSL 907 0.945 1.160 0.976 Male TL = a + bSL 230 0.423 1.190 0.980 SL = a + bFL -0.229 0.971 0.990 SL = a + bFL 0.068 0.955 0.991 FL = a + bTL -0.029 0.862 0.974 FL = a + bTL -0.025 0.862 0.987 Female TL = a + bSL 791 1.012 1.165 0.961 Female TL = a + bSL 644 0.315 1.199 0.972 SL = a + bFL -0.102 0.962 0.990 SL = a + bFL 0.448 0.937 0.969 FL = a + bTL 0.055 0.856 0.965 FL = a + bTL 0.297 0.848 0.961 Combined TL = a + bSL 1,698 0.885 1.167 0.969 Combined TL = a + bSL 874 0.301 1.199 0.975 SL = a + bFL -0.137 0.965 0.990 SL = a + bFL 0.356 0.941 0.974 FL = a + bTL 0.049 0.857 0.971 FL = a + bTL 0.233 0.850 0.967

Figure 3. Monthly averages of Fulton’s condition factor for Arius dispar from Laguna de Bay, Philippines. No. of specimens: males = 907; females = 791. Error bars represent standard error of the means.

Figure 4. Monthly averages of Fulton’s condition factor for Arius manillensis from Laguna de Bay, Philippines. No. of specimens: males = 230; females = 644. Error bars represent standard error of the means

91 Philippine Journal of Science Quilang et al.: Length-Weight and Vol. 146 No. 1, March 2017 Gonadosomatic Index of Arius species

Figure 5. Monthly averages of gonadosomatic index for Arius dispar from Laguna de Bay, Philippines. No. of specimens: males = 907; females = 791. Error bars represent standard error of the means.

Figure 6. Monthly averages of gonadosomatic index for Arius manillensis from Laguna de Bay, Philippines. No. of specimens: males = 230; females = 644. Error bars represent standard error of the means.

The b values computed in this study fall between 2 and 4 comprehensive study on the LWR of this commercially and are close to 3; the values that are usually obtained for important fish species in the Philippines. To date and to fishes (Tesch 1968). These values usually differ between our knowledge, there is no published or unpublished study species, between stocks or populations within species, yet on the LWR of A. manillensis. between different stages of development, between sexes, or depending on differences in environmental conditions Only 17.7% of A. manillensis and 15.2% of A. dispar (Le Cren 1951; Tesch 1968; Froese 2006). The r2 values consist of mature specimens. This estimate may have for LWR are relatively high, which shows that the length been affected by sampling error, since some female specimens with SL lower than L were observed to have measurements are good determinants of weight. m ripe gonads. Nonetheless, the presence of immature There is only one published study on LWR of A. specimens is evident from the sample. Likewise, majority dispar, but the regression equation was based only on of the specimens collected were below the margin three individuals collected from Candaba wetland in of optimal yield. These are indicators of overfishing the Philippines (Garcia 2010). Hence, this is the first (Froese 2004).

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Condition factor was observed to vary significantly species, more than half were smaller than Lopt-10%. Last, between species, between sexes, and between different individuals above Lopt+10% are deemed mega-spawners months. Differences in condition factor could be attributed and should also be excluded from the catches. Although to variability in fatness, suitability of the environment or low in frequency, large specimens were also present in stage in the development of the gonad (Le Cren 1951). GSI the samples. These results show that these species are for both species was much higher from April to September still overexploited. Aside from regulating the sizes of fish 2010 compared to the other months, although there was a included in catches, it is also important to consider the marked depression in the month of June. Mature ovaries month of the year and ensure maximal spawning occurs. and large ova were observed during the peak months, Results from this study show that spawning peaks from which could be the spawning runs for both species. The February to May (dry season) and from July to September first period corresponds to the summer months, while the (wet season). If seasonal closure during the peak months second one corresponds to the onset of the rainy season. is not possible, the mega-spawners and extremely gravid The occurrence of two spawning periods is not surprising females could be excluded from the catch. These regulatory as these Ariidae species are batch spawning fishes (Mane measures have to be enforced by local governments and 1929). Results of the present study also confirm the concerned government agencies to prevent these valuable account of Aldaba (1931) that based on the observations fishery resources from being depleted again. of fishermen, spawning of kanduli takes place during the height of the dry season, from February to May, with the greatest spawning activity occurring in March and April. For A. manillensis, Mane (1929) reported that spawning ACKNOWLEDGMENTS lasts from middle of February to the first week of May and This study was funded through a PhD Incentive Grant that during the breeding season, spawning runs take place (Project No: 090921) awarded to J.P. Quilang by the Office during the middle of February, middle of March, middle of of the Chancellor of the University of the Philippines April, and first week of May. Mane (1929) inferred these Diliman through the Office of the Vice-Chancellor for four spawning runs by measuring the diameter of 300 ova Research and Development. Special thanks also to the from 10 fish examined each month from September 1927 to Institute of Biology, University of the Philippines Diliman June 1928. No measurements were made by Mane (1929) for logistical support. for the months of July and August. In this study, for both A. manillensis and A. dispar, in addition to the dry months (February to May), July and August are also peak months for spawning. In other Siluriform species, the reproductive REFERENCES period draws its cues from the amount of rainfall (Liang et al. 2005; Hossain et al. 2006). Although these involve only ALDABA VC. 1931. The kanduli fishery of Laguna de one spawning period throughout the year, there are other Bay. Philipp J Sci 45:29-39. fish species that exhibit biannual spawning period (Hussain CONLU PV. 1986. Guide to Philippine flora and fauna. and Abdullah 1977; Gill et al. 1996). Vol IX. 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