Work Package 5: Fisheries and Population Dynamics

PHILIPPINES   

The problem of declining populations of Scylla spp. can be addressed through regulation of fishing effort, rehabilitation of mangrove habitats, and enhancement of wild crab stocks aside from seed production in hatcheries. 

  Buswang, Kalibo Mangroves  

One of the most successful reforestation schemes has been the 57-ha Buswang Mangrove project in Kalibo, Aklan province. In cooperation with the local government and a local NGO, the cooperative formed by local villagers called KASAMA (Kalibo Save the Mangrove Association) initially planted an area of 45 ha of Rhizophora sp. and 5 ha of Nypa fruticans, and later another 7 ha of mangroves. A more detailed history of the project is given by Ashton (2001) and Primavera (2002 a and b).

Mud Crab Fishery

The mud crab fishery was continuously monitored from March 2002 to December 2003. Fishermen used bamboo traps ('tapangan') and baited lift nets ('bintol') . A third collecting gear was the stake net ('bakikong') with stretched mesh size 3 cm which covered a drainage creek that caught crabs on the retreating tide.

Bamboo Traps

Baited lift nets

Stake net

Of the 10,707 crabs recorded over the 21-mo period, 10,480 were caught inside the mangrove by mobile 'tapangan' and 'bintol' traps. The crabs were identified as S. olivacea (99.33 %), S. tranquebarica (0.6%) and S. serrata (0.07%). The 'bakikong' caught only 226 crabs comprising 71.3% S. olivacea, 28.3% as S. tranquebarica and 0.04% as S. serrata, suggesting that S. olivacea is more dependent on the mangroves than the two species. 
Monthly size-frequency plots show no obvious modal progression suggesting that recruitment maybe continuous (click here to Figure 1). Males made up of 57% of the population and dominated in the smaller and larger size classes. Seventy five percent of the population was between 5-9 cm carapace width (CW), and 55% were sexually immature. Their removal prior to sexual maturity may have important repercussions on the population dynamics. The limit to the smallest sizes caught may be an artifact of the size selectivity of the type of traps used, or reflect the actual recruitment size of crabs into the mangrove population. Various methods were used to catch smaller size classes, e.g., smaller bamboo traps, habitat traps in the form of roof tiles, and 4mm mesh nets were dragged across different habitats within the mangrove and on the fringes but all to no avail suggesting that smaller crabs probably live in the sub-littoral outside the mangroves. The upper limit may reflect the migratory patterns of the larger crabs moving offshore, or natural mortality.

The fairly constant proportions of males and females suggest that there is no seasonal spawning migration by females, and gravid females were present through out the year. Only 7 berried females were caught, 4 within the mangrove and 3 in the barrier net suggesting that females probably spawn locally as supposed to offshore during the dry season when salinity is high and that some berried females continue to migrate back to the mangroves to feed during spring tides.

A significant regression analysis (p <0.001, r2 = 51.7%) of mean CW against time suggests that CW has significantly increased from 6.5 to 7.6 cm in both 'tapangan' and 'bintol' catches over the 21-mo period. Mean body weight (BW) has correspondingly increased from 61 g to 96 g. However, mean catch per unit effort (CPUE) as measured by the number of crabs caught per trap per tide, has decreased. In 'tapangan' where the soak time is constant, CPUE has decreased from 1.71 crabs/trap/tide in April 2002 to 0.85 crabs/trap/tide in November 2003 (p <0.001, r2 = 30.9%).

Conventionally falling CPUE and declining mean capture size are regarded as classic indicators of over fishing. Falling CPUE and rising mean capture size in the Buswang mangrove fishery is confusing. However, the falling CPUE indicates the decreasing density of mud crabs within the mangrove with fewer smaller crabs being caught -- this may reflect a problem with the recruitment of smaller crabs.

Females were found to mature at 8.44 cm CW; majority of the crabs caught were below this size. There was no significant correlation between the CPUE and lunar phase or time at which high tides occur (day or night), or the area in which they were caught. A Kruskal-Wallace test suggests that the New/Full Moon or the occurrence of high tide during the day/night did not influence the presence of gravid females.

In summary, the Buswang mangrove fishery is dominated by S. olivacea and declining mean CPUE indicates the population may be overexploited.

  Bugtongbato-Naisud, Ibajay Mangroves  

Of the 3,243 crabs caught during spring tides from April 2002 to December 2003, 96.2% were Scylla olivacea, 0.2% S. serrata and 3.6% S. tranquebarica. Of the 1,738 crabs caught by 'tapangan', 95.2% were S. olivacea, 0.4% S. serrata and 4.4% S. tranquebarica. The 'bintol' had the lowest number of crabs caught (n=338) with 96.7% S. olivacea, 0.3% S. serrata and 3.0% S. tranquebarica. The 1167 crabs caught by baited line with scoop net or 'tonton'  were 97.4% S. olivacea and 2.6% S. tranquebarica.

Scoop net

Males comprised 47% of the population while 53% were females. From April 2002 to July 2003 (except February 2003) there were more females than males with a peak in October 2002. From August to December 2003, samples were dominated by males with a peak in October 2003. Comparing the gears, 'tapangan' had more female catches only in 5 (April, May, June, August 2002 and February 2003) of 21 months sampling. However, both 'bintol' and 'tonton' had more females most of the time while more males were obtained in August 2002, February, April, October and November 2003 for 'bintol', and April 2002, August, October and November 2003 for 'tonton'. This may suggest sex selectivity of gear but this needs to be compared with the results from Kalibo.

Mean size of crabs showed two peaks in September-October 2002 and August and October 2003. Mean CW was significantly highest (a=0.05) for crabs caught by 'tonton' (8.0 cm; range = 3.0-19.5 cm), followed by 'bintol' (7.6; range = 3.0-11.5 cm) and then 'tapangan' (7.3; range = 3.0-12.0 cm).

There was an abrupt drop in CPUE (both for number and biomass) from April to October 2002. A stable CPUE of 0.46 crab/gear/day and 46 grams/gear/day on the average was observed from November 2002 to December 2003. In terms of number, 'tonton' had the highest CPUE, followed by 'bintol' and 'tapangan'. However, highest CPUE for biomass was 'tonton', followed by 'tapangan' then 'bintol'. All 3 gears showed a declining CPUE in number and biomass from April to September 2002. However, 'bintol' CPUE was higher in most of the months from November 2002 to December 2003, while 'tapangan' and 'tonton' had almost the same values. This was due to fewer number of 'bintol' deployed compared to 'tonton' and 'tapangan' with the hiring of a new 'bintol' fisher starting November 2002.

Over six spring tides from February 2004 to April 2004, 825 Scylla spp (98.6% S. olivacea, 0.6% S. serrata, and 0.8% S. tranquebarica) injected with coded microwire tags were released in four major branches of Naisud River. Of these, 22% have been recaptured over five mark-recapture sessions starting in February (Table 2). Recaptures will be monitored starting May 2004.

 

Table 2.

Summary of the number of crabs tagged, released and recaptured in Naisud-Bugtong  Bato, Ibajay mangroves (February-April 2004)

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