Replenishment and connectivity of reef fish populations in the central Philippines
Abesamis, Rene A. (2011) Replenishment and connectivity of reef fish populations in the central Philippines. PhD thesis, James Cook University.
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Abstract
The dynamics of reef fish populations at lower latitudes are not well understood, particularly in the Coral Triangle (composed of the Philippines, Malaysia, Indonesia, Timor Leste, Papua New Guinea and the Solomon Islands), where the monsoons (shifting tradewinds) are potential drivers of seasonality of reproduction and recruitment. Such dynamics will also be influenced by the degree to which local populations self-replenish or rely on larval replenishment from other populations. However, few have investigated the effects of the monsoons on the replenishment of reef fish populations. Furthermore, the extent of ecologically-significant larval dispersal of reef fishes, in general, remains unclear. Understanding the patterns and key processes behind population replenishment and larval dispersal are critical to the application of no-take marine reserve networks as conservation and fisheries management tools. This thesis addresses knowledge gaps about the patterns of reproduction, recruitment and larval connectivity of reef fish populations in the central Philippines, the epicentre of reef fish biodiversity in the Coral Triangle.
The questions that this thesis addresses are especially relevant to the use of notake marine reserves for reef conservation and fisheries management in the Philippines, a developing archipelagic nation with a vast coral reef area, a large and rapidly growing human population and a great number of small, community-based marine reserves. Most coastal communities in the Philippines are poor and have few employment alternatives. Many communities rely on coral reefs for food and livelihood. No-take marine reserves in the Philippines are regarded as a viable option to conserve and manage coral reefs that is acceptable to local stakeholders.
This thesis consists of four related studies. In the first study, I tested the prevailing notion that spawning peaks of reef fish at lower latitudes are timed to take advantage of weaker winds during inter-monsoonal periods, supposedly to limit advection and enhance survivorship of larvae. The monthly spawning patterns of four species of reef fish (2 fusiliers, 1 surgeonfish and 1 damselfish) were studied for 11-22 months. All four species showed protracted spawning periods. However, results suggested that the monsoons affect reproduction in ways that are not consistent with the above hypothesis. Conditions advantageous to larval survival may not be restricted to the inter-monsoonal periods. An alternative explanation is that the spawning patterns probably reflected temperature, rainfall, wind or wave action more directly influencing the spawning of adults, as opposed to adaptation of the timing of reproduction to ensure higher survivorship of larvae in the pelagic environment. The complex effects of the monsoons on reproduction in reef fishes warrant further study.
In the second study, I investigated the patterns of recruitment (larval settlement) of 120 species of reef fish almost every month for 20 consecutive months at two island and two coastal locations. Recruitment was found to occur throughout the year. Most species exhibited protracted recruitment seasons (up to 9-11 months). However, a predictable annual peak in community-wide recruitment of reef fishes was detected, coinciding with the weakening of monsoon winds and higher sea surface temperature. The annual pattern of recruitment was fairly consistent across 11 sites and between the two years sampled. The same recruitment pattern was also found in the two families that dominated monthly recruitment to reefs, the damselfishes and the wrasses, notwithstanding a 10-fold difference in overall abundance of recruits of these two families. These findings implied a far-reaching influence of the monsoons on recruitment of reef fishes at lower latitudes.
In the third study, I determined the extent of potential larval connectivity among reef fish populations across a 300-km-wide region of the Bohol Sea in the central Philippine archipelago, by combining two very different methods: 1) analysis of species assemblage patterns (presence/absence of 216 species at 61 sampling sites) and associated habitat patterns; and 2) modelling of reef fish larval dispersal patterns. The results of the two methods independently suggested probable connectivity within a large group of sites situated in an internal sea where a dominant westward current is present. The presence of potentially significant connectivity among these sites, and their lack of strong connectivity with other sites were probably strongly influenced by the local oceanographic setting and habitat. The results were consistent with present knowledge of the spatial scales of ecologically-relevant larval dispersal in reef fishes (10's of kilometres). The study provided a framework for connectivity within existing networks of no-take marine reserves in the region.
In the final study, I estimated the probable extent of recruitment subsidy (net larval export) and larval connectivity of 39 small (< 1 km2) no-take marine reserves within a more limited (135 x 70 km) region of the Bohol Sea in the central Philippines. The reserves were estimated to occupy about 6% of total reef area in that region. I used a simple exponential model of population recovery to estimate the increase in larval production of large predatory reef fishes inside reserves over time. Larval dispersal and recruitment were simulated using a larval dispersal model. The model simulations showed that a 3.5-fold (~250%) increase in recruitment to fished areas may result from a 55-fold increase in larval production inside reserves if all reserves were effectively protected for 20 years. Lesser larval subsidies are likely to more than replace losses in yield due to reserve creation but they will be difficult to detect because of temporal and spatial variation in recruitment. The strength of larval connectivity between reserves increased dramatically (up to ~20-fold) with greater larval production. These findings highlight the importance of protecting reserves over the long-term (decades) and establishing reserve networks that interact effectively via larval exchange in order for reserves to sustain reef fisheries in highly overfished areas and the reserve networks of which they are a part.
A synthesis of the results of this thesis highlighted the importance of the monsoons and local geographic setting in shaping the patterns of replenishment and connectivity of reef fish populations in the Coral Triangle. The implications of potential relationships between the monsoons and the dynamics of reef fish populations amidst a changing climate were also explored. Finally, future research that can validate ecologically-relevant larval connectivity within reserve networks and encourage the long-term protection of reserves were proposed.
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