Demographic patterns in coral reef fishes: biogeographic trends and fishing effects

Ranatunga, R.R.M.K.P. (2007) Demographic patterns in coral reef fishes: biogeographic trends and fishing effects. PhD thesis, James Cook University.

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The present study is a multi-scale study which investigates the demographic plasticity within and between geographically distinct reef fishes of the families Lutjanidae (Snappers), Serranidae (groupers) and Labridae (hogfish). This also investigates how fishing alters fish demography and demographic shifts associated with introduction to new habitats. These were determined by analyzing age-based demographic parameters, biological and ecological information as a basis for management and conservation. Demographic characteristics such as size, age, longevity, mortality rates and broad-scale spatial patterns in growth were described in this comparative study. Present study will provide unique contribution to general life history theory by focusing on number of species and large scale environmental gradients extending latitudinally and longitudinally across and within Caribbean, Pacific and Indian oceans.

The potential shifts in demography of Lutjanus griseus (gray snapper) was examined across a geographic gradient. Samples were obtained from locations spanning from low latitude Los Roques (11°N), and Belize (17°N) to high latitude Bermuda (32°N) in the Caribbean. Results revealed substantial plasticity in demography across latitudinal gradient within and among species. Maximum longevity for L. griseus varied from 6 yr in Belize, 12 yr in Los Roques to 34 in Bermuda. Longevity showed almost 5-fold difference in Bermuda compared to its Belize counterpart. Maximum age of 34 yr determined in this study is the oldest age recorded for L. griseus, anywhere in the world. Maximum size also showed great degree of variation among location. Largest maximum size of 770 mm for L. griseus was found in Bermuda where as smallest maximum size was 440 mm in Belize. This large body size and extended lifespan of Bermuda population presumed to be latitudinal effect rather than fishing effect. Maximum longevity showed a strong negative relationship with sea surface temperature. This support the idea that there are fundamental differences in ecology, biology and population dynamics of reef fish assemblages over bio-geographic scales.

Present study also investigated the age-specific growth and life-history characteristics of hogfish, Lachnolaimus maximus, a large reef fish belongs to the family Labridae (wrasses). Fishing pressure on hogfish has reduced many populations of hogfish to critically low levels and the species has identified as vulnerable to extinction and included in IUCN Red List. L. maximus examined from four geographic locations in the Caribbean subjected to varying levels of fishing pressure. Hogfish from Margarita and Belize, locations with high fishing pressure, had a smaller FL (mean FL of 239 and 278 mm, respectively). Los Roques, where the reefs are protected from fishing has the largest mean FL (459 mm). Maximum longevity observed for L. maximus was 16 yr from Bermuda and 10 yr in Los Roques. Margarita and Belize populations were relatively short-lived with the oldest fish estimated were 5 yr and 7 yr, respectively. The results also demonstrated that hogfish collected from Bermuda and Los Roques attained a larger size-at-age than those fish sampled from Belize and Margarita. Growth curves indicated that L. maximus from Belize and Margarita were slower growing compared to two other populations making them more vulnerable to exploitation. Estimates of the instantaneous rate of total mortality for hogfish showed lower values in Los Roques (0.45) and Bermuda (0.53), in contrast, intensively fished populations showed significantly higher total mortality rates [Belize (0.81) and Margarita (0.71)].

Lutjanus apodus from Belize and Los Roques, and Cephalopholis cruentatus from Belize, Los Roques, Las Aves, Curacao and Barbados, locations from the same latitude but subjected to varying levels of fishing, were studied for possible demographic signatures of fishing. Growth curves of L. apodus showed significant differences among two locations indicating different growth trajectories. Both Belize and Los Roques populations had similar growth up to 6 years and noticeable shifts in L∞ in Los Roques thereafter. 95% Confidence regions around K and L∞ showed no overlapping between Belize and Los Roques which confirmed the existence of different growth trajectories. Among L. apodus populations, mean maximum sizes achieved were higher in Los Roques compared to Belize (510 and 381 mm, respectively). Belize population of L. apodus had higher instantaneous mortality rate (0.17) compared to Los Roques (0.08). Both Belize and Los Roques locations lie within the same latitude minimizing any latitudinal effects. Therefore differential growth trajectories presumed to be associated with different exploitation rates exert on the two populations. The smaller size-at-age observed in Belize could be a result of high fishing pressure and therefore, removal of older age classes. The maximum age achieved for L. apodus in present study was 31 years from a protected reef environment (Los Roques) while maximum age recorded from a location subjected to fishing (Belize) was 24 years. Maximum longevity observed in present study for L. apodus (31 yr) is the highest recorded from anywhere in the world. It was evident that L. apodus has remarkably greater longevity than previously reported. Six populations of Graysby (Cephalopholis cruentatus) subjected to three levels of fishing, (high fishing - Belize and Curacao, moderate fishing - Las Aves and Barbados and low fishing - Los Roques) were also evaluated for their resilience to fishing. These populations were found along same latitude. Graysby population from Los Roques, where fishing is not allowed, had extended lifespan (22 yr). Populations from Las Aves and Barbados had moderate level of fishing intensity and also moderate maximum lifespan, whereas in Belize and Curacao with highest fishing intensity, therefore, highest mortality rates (0.5 and 0.6), had very short maximum lifespans (8 yr in both locations). Belize and Curacao showed no obvious asymptote although all the other populations did so. The Curacao and Belize populations exhibited a pattern of exponential decline in age distribution without marked peaks thereafter, while other populations had multi-mode age distributions. Both Belize and Curacao populations showed slow growth making them more vulnerable to intense fishing. It seems that high fishing pressure influences age structure, resulting in shorter lived fish.

The demographic shift associated with introduction to new environments was investigated as the final part of this study. The Hawaiian Archipelago is known for its relatively less-represented nearshore marine fish fauna. The most striking feature is the absence of native shallow-water snappers and groupers, two most common taxa found in shallow reefs elsewhere in tropics. The blacktail snapper, Lutjanus fulvus has been introduced to Hawaii from their native habitats of Moorea and Marquesas Islands, where they become established in Hawaiian Archipelago. These introductions have implications for conservation biology because, in spite of the fact that only a few individuals transfer their characteristics to subsequent generations, no significant change in genetic diversity can be observed. As L. fulvus was introduced into the Hawaii reefs with no native snappers or groupers, it was expected that they would have a rapid growth rate compared to Moorea. However, the Marquesas is a high productivity environment due to upwelling which may result in very rapid growth. Results revealed that L. fulvus in Hawaii did not attain ages as great as none of their native populations in Marquesas or Moorea. Longest-lived L. fulvus were found from Marquesas 10 yr and Moorea (7 yr), and shortest-lived was the Hawaii population (6 yr). von Bertalanffy growth curves revealed that Hawaiian population grew faster than their native populations and attained a larger maximum size. Relative absence of competitors in Hawaii would be the reason for this larger body size and faster growth. These Pacific populations were also compared with L. fulvus from Cocos (Keeling) (Indian Ocean). The 95% confidence ellipse revealed that the demographic characteristic of Hawaiian population of L. fulvus was more similar to Cocos (Keeling) than their ancestral populations in Marquesas or Moorea.

Based on the studies I conclude that there are obvious biogeographical scale patterns in demographics and the differing patterns among and within species indicate complex spatial variability in demographics. Reduced maximum size with corresponding reduction in average age and longevity may be a result of latitudinal effect or fishing effect. Exploitation influences community structure directly through preferential removal of larger-bodied fishes or indirectly as, larger-bodied fishes may exert top-down control upon other community members. Some of the species concerned shared some characteristics associated with high vulnerability to fishing, e.g. relatively slow growth and moderate to long lifespans, demography and body size. Estimates of mortality and survival rates for the selected species indicate the present status of exploited populations and also give insight on how long this fishery could be sustain under present fishing pressure. This study also provides the opportunity to examine species that have recently invaded areas from which they were naturally absent. This will contribute significantly to the knowledge of the effects of introductions into the marine tropical water and improve the basis for management of such actions.

Item ID: 29824
Item Type: Thesis (PhD)
Keywords: coral reef fishes; demography; Caribbean; Hawaiian Archipelago; bio-geographic gradients; spatial variability
Date Deposited: 18 Oct 2013 05:13
FoR Codes: 06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060303 Biological Adaptation @ 40%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 20%
06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060308 Life Histories @ 40%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 50%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 50%
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