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Epinephelus Genus Groupers Population Structure and Length-Weight Relationships off Kenyan South Coast Indian Ocean Marine Waters

B.O. Ogongo, E.C. Mwachiro and B.M. Fulanda
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Groupers are apex predatory fishes playing important local economic roles and ecosystem functions worldwide. The Kenyan coast Indian Ocean waters have been a good landing centre for nearly 30% of the already identified 98 Epinephelus genus species groupers. However, of late, their natural populations have greatly decreased with the landed individuals mean length and weights showing drastic reductions. Most are therefore caught when either juveniles or sub adults. Information data is therefore needed to avert this alarming decline and initiate conservation measures for these prized premium fishes. This study elucidates the Epinephelus genus groupers population structures and length-weight relationship analyses for these highly sought fishes from Kenyan inshore marine waters. The study results indicated varying significant conditions (p<0.001) for these species having Length-Weight Relationship (LWR) regression coefficients (R2) that appeared to be stronger (R2>0.6000) for 23 species. Despite this apparently strong fit, Analysis of Covariance (ANCOVA) revealed no significant differences between (a) intercept and (b) slopes of the species regression lines (p = 0.5552). R2 analyses also showed consistent non isometric patterns in the plotted power curved data suggesting that unless stringent management measures are implemented to restrict over-exploitation, it is likely that the species currently categorized as ‘Near Threatened’ under the International Union for Conservation of Nature (IUCN) Red list would soon become ‘Endangered’ while those ‘Endangered’ extinct. This can greatly alter the ecosystems’ ecological health as they are apex predators in the demersal food web and their stock depletions would result in the alteration of the ecosystem.

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B.O. Ogongo, E.C. Mwachiro and B.M. Fulanda, 2017. Epinephelus Genus Groupers Population Structure and Length-Weight Relationships off Kenyan South Coast Indian Ocean Marine Waters. International Journal of Oceanography and Marine Ecological System, 6: 1-8.

DOI: 10.3923/ijomes.2017.1.8

Received: July 26, 2015; Accepted: September 02, 2015; Published: December 15, 2016

Copyright: © 2017. This is an open access article distributed under the terms of the creative commons attribution License, which permits unrestricted use, distribution and reproduction in any medium, provided the original author and source are credited.


Marine biodiversity loss threat is increasing worldwide due to overfishing, climate change and habitat destruction. This vulnerability threat has been more apparent to fishes because of the changing fishing patterns; excessive fishing of unwanted by-catch; illegal fishing; excessive fishing of the fishes before attainment of first maturity lengths; use of fishing techniques that harm both the environment and fish stocks among other factors. Conservation of vulnerable and threatened fish species has thus gained great global ecological importance over recent years. As a result, the fish’s life-history characteristic knowledge is of great importance for effective implementation of sound conservation management strategies1-3.

The Kenyan Indian Ocean marine waters homes 30 species apposite for human utilization out of the 98 known Epinephelus genus grouper species3-5. These genus groupers belong to the Epinephelidae subfamily in the Serranidae family and are widely distributed in Hawaii waters, Indo-west Pacific, Red Sea, Philippines, Southern Japan and in Eastern Africa where they are popularly known as groupers or rock cods5. Majority are long lived, slow growers and have lower fecundities in addition to forming spawning aggregations which are heavily exploited by both small and large-scale fishers6-9.

The fishes constitute important components of the demersal fishery resources that are main targets of commercial fishers in both tropical and sub-tropical waters worldwide. As a result, the genus fishes are important components of the inshore small-scale fisheries along the Eastern coast of Africa where they are locally known as "Tewa" or Rock cods. However, a recent10 United Nations Environmental Programme (UNEP)-Nairobi Convention in conjunction with Western Indian Ocean Marine Science Association (WIOMSA), reported that an estimated 1,900 bony fish species including some Epinephelus genus groupers are threatened within the region11. Despite this threatened status, they are still target species of the rural fisher folks within the Western Indian Ocean (WIO) region for income, fish protein and micronutrients. This, in addition to various ecological and environmental degradation changes of the natural habitats within the region, has resulted in serious population declines with some almost being driven to the verge of extinction11.

The current global rate of grouper exploitation and mortality is also arguably the highest under current management regimes that unless a renewed initiative is undertaken, some species will become effectively extinct2 and creating shifts in the biological communities with resultant systemic biodiversity losses12. Thus, given the vulnerability of the fishes even to modest fishing efforts due to their slow growth rates, late maturity, low fecundity and long life spans, potential recovery of their populations to overharvesting can take decades and human decisions regarding them is a crucial element of marine-related social science13.

A fish’s Length-Weight Relationship (LWR) is an important fishery management tool in fisheries biology and stock assessments in all water bodies as it enables the determination of fish age, structure and health by providing various facts about its seasonal cycles and influential aspects of the biotic and abiotic factors14. This is because an average fish weight of a given length group can easily be estimated by establishing a mathematical relation between length and weight parameters15. They also show an indication of the fish’s gonad development and are useful for regional comparisons and specific species histories16. In addition, the data also provides important clues on climate, environmental and human subsistence changes16,17 that help in predicting the best suited time and length for harvesting particular fish species18,19.

Therefore, with no information existing currently on the Epinephelus genus groupers’ population structure and length-weight relationships off the 97 entire Kenyan Indian Ocean Marine Waters, the study presents the first study on population structure and length-weight relationships of these vulnerable fish species in Kenya using a one year collected length and weight morphometric data.


The study was conducted within the inshore small-scale marine fishing areas of Msambweni (S04°30'31.26", E039°28'12.97") lying about 70 km south of Kenyan Mombasa city; Shimoni (S 04°38'52.67", E 039°22'50.55") of which part of its traditional fishing grounds has been hived off and gazetted as a marine protected area under the Kisite-Mpunguti Marine National Park and Reserve and Vanga (S04°40'16.11", E039°13'36.96") which its fishing grounds and village is situated on the southern tip of Kenya-Tanzanian border in the Kenyan south coast (Fig. 1). The study sites were selected based on the existence of previous grouper studies undertaken from there; their high grouper fishers and fisheries activity coefficient within south coast of Kenyan and lastly, for being the most pristine and sheltered reefs on the Kenyan coast making them key grouper fishing areas.

Image for - Epinephelus  Genus Groupers Population Structure and Length-Weight Relationships off Kenyan South Coast Indian Ocean Marine Waters
Fig. 1: Study landing sites map of Vanga, Shimoni and Msambweni

Field sampling and laboratory work was conducted from December, 2013 to November, 2014. The Epinephelus genus grouper specimens were purchased from the small-scale artisanal fishery catches landed at the sites. Purchased freshly landed catches were immediately chilled in ice on site and transported to Kenya Marine and Fisheries Research Institute (KMFRI) Mombasa Laboratory for analyses. At the laboratory, the specimens were sorted and identified to species level using keys and guides from Anam and Mostarda4 and Heemstra and Randall5. The already identified specimens Total length and Weights were then individually measured and recorded. Total Length (TL) was measured to the nearest 0.01 cm using digital slide calipers (Mitutoyo, CD-15PS) and total body weight (W) was measured using an electronic balance (Shimadzu, EB-430DW) with 0.01 g accuracy. All morphometric measurements were conducted according to Froese and Pauly20. Length and sex-ratio distributions were also constructed separately for the species specimens and only those with more than one individual landing were used for sex-ratio determinations. Each species lengths were categorized from the smallest to the largest to determine the existing ranges. The isometric (b = 3) or allometric growth relationship between total length (TL, cm) and total body weight (W, g) was described for these fishes growing with their bodies becoming heavier using a plotted power function; W = a*TLb in which a is the power function coefficient (the regression intercept) and b the exponent (the regression slope). The relationships were estimated by linear regression analyses based on natural logarithms: ln (W) = ln (a)+b ln (TL). Prior to the analyses, ln-ln plots of length and weight values were performed for visual inspection of outliers in accordance to Froese21. Growths’ were considered positively allometric if the estimate of b was approximately equal or greater than 3 and negative if less than 3. Fulton’s condition factor (KF)22 was calculated using the equation:

KF =1000×(W×L–3)

where, W is the total body weight (W, g) and L the total length (TL, cm). The scaling factor of 1000 was used to bring the KF close to unit for all the species.

The non parametric Chi-square goodness-of-fit test was used to test for significant deviations from the expected 1:1 male to female ratio and also to compare the species numbers within the study sites using MINITAB release 14 software. In addition, the plotted LWRs were power tested using the coefficient of regression (R2) to determine significant differences from the isometric value of b = 3 using Microsoft® Excel-add-in-Daniel’s XL Toolbox spreadsheet computer package. The a and b parameters of the species LWRs were also compared by covariance (ANCOVA) analysis and all statistical significance tests determined at α = 0.05.


The species population abundance of the encountered thirty Epinephelus genus groupers were generally lower and had skewed distribution patterns within the study sites. As a result, some species were not landed in certain sites and where landed, were less than 5 individuals. However, significant difference (p-value = 0.030) existed on the total distribution abundance of all pooled landed specimens at the sites (Table 1). Their lengths, weights and sexes were also as tabulated in Table 2. The obtained length and weight data for 25 species with more than two individual landings were plotted and power tested using regression coefficients of determination (R2) to determine if their length-weight growth relationships were isometric (b = 3), or allometric (b<or>3). All twenty five of the species had negative allometric growth relationships where b<3 with only two species; one large bodied species, E. fuscoguttatus (with maximum adult growth length size of 120 cm) (Fig. 2a) and one small bodied species, E. miliaris (with maximum adult growth length size of 43 cm) Fig. 2b) having allometric growth relationships where b ≈ 3 (Table 3). The species calculated Fulton’s condition (KF) factor values also showed significant variations (p<0.01) with the best and worst performers being E. spilotoceps with 127.551 and E. epistictus (2.856), respectively (Table 3).

Table 1:Landed pooled genus species abundance and distribution patterns within sites
Image for - Epinephelus  Genus Groupers Population Structure and Length-Weight Relationships off Kenyan South Coast Indian Ocean Marine Waters
aN: Numbers landed, p-value (probability of significance at α = 0.05)

Table 2:Landed genus species numbers, lengths (cm), weights (g) and sex data
Image for - Epinephelus  Genus Groupers Population Structure and Length-Weight Relationships off Kenyan South Coast Indian Ocean Marine Waters
Image for - Epinephelus  Genus Groupers Population Structure and Length-Weight Relationships off Kenyan South Coast Indian Ocean Marine Waters
F: Female, M: Male, Min: Minimum, Max: Maximum, TL: Total length, SL: Small length

The observed minimum total length among the 401 sampled specimens ranged from 13-31 cm for the small and medium sized species with majority being slightly above 22 cm. The mobile and larger bodied species on the other hand sizes ranging from 17.4-77.9 cm with most being slightly above 57 cm (Table 2). However, secondary maturity data obtained from published literature was used to determine if the landings comprised of immature fishes since the present study’s data could not allow estimation of first maturity sizes.


The Kenyan IUCN red list threatened fishes biometric information has been quite insufficient despite a number of studies having been conducted within the Asian countries9,18,19,23-28. In this study, the size structures of 17 species (E. areolatus, E. coeruleopunctatus, E. chlorostigma, E. coioides, E. fasciatus, E. hexagonatus, E. longispinis, E. macrospilos, E. malabaricus, E. melanostigma, E. merra, E. miliaris, E. multinotatus, E. poecilonatus, E. spilotoceps, E. tauvina and E. tukula) demonstrated some marked length size class range differences (Table 3). The differences may be particularly attributed to environmental water temperature and food availability variations29 thus signaling the need for urgent extensive studies on the entire genus species to provide more management and conservation information.

Image for - Epinephelus  Genus Groupers Population Structure and Length-Weight Relationships off Kenyan South Coast Indian Ocean Marine Waters
Fig. 2(a-b):
Total length-Weight relationship for (a) Epinephelus fuscoguttatus and (b) Epinephelus miliaris

Table 3: General species length-weight relationship (LWR) and condition estimates
Image for - Epinephelus  Genus Groupers Population Structure and Length-Weight Relationships off Kenyan South Coast Indian Ocean Marine Waters
N: Numbers of the landed individual species, R2: Regression coefficients of determination, KF: Fulton’s condition factor value

Overall, the species catches and abundance declines are suspected to have resulted from increases in fishing effects and effort due to increased demand for food. It could have been also a reflection of different species settlement histories as evidenced by their modal size class structure differences that were mostly below the reported most groupers first maturity sizes30,31. Conversely, the observed species dominance and distribution may have been also due to their ability differences to deal with extreme physical site conditions. This was evidenced by the non-significant distributions between the small and medium sized (χ2 contribution = 2.055; 1 df; p-value = 0.416) and the larger sized mobile groupers (χ2 contribution = 4.926; 1 df; p-value = 0.085) (Table 1).

Nearly all species of fish LWR is expressed by the equation: W = a*Lb and the change in weight can be described by the relationship. Thus, in cases where the relationship regression slope b<3 or >3, growth is said to be allometric and the fish becomes less rotund when b<3 or more rotund when b>3 with increasing length. However, when b = 3, growth is said to be isometric and the fish grows with unchanged specific gravity and body proportions. It is however also possible for shape to change when b = 3 as a result of changes in the regression intercept "a"22,32 making the estimated LWR parameters also to differ among seasons and years due to physicochemical characteristics of the environment, sex and maturity stages of a given fish species. This isometric relationship departure may however be minor for some early life history aspects but may become more important in the calculation of metabolic processes as was stated by Cone22 and Laurence33. As a result, fish condition studies assume that heavier fishes are often of better conditions and condition indices have been frequently used by fish culturists as indicators of the general population ‘well-being or fitness’.

This study’s results therefore provide the much needed information required for future comparisons and urgent detection of any long-term declines in the conditions that may have occurred as a result of environmental degradation, key physiological components of the fish’s life history and growth.


This study provides important baseline information for the Kenyan Epinephelus genus groupers LWR and conditions needed by fishery biologists, managers and conservationists’ for initiating early management strategies and future studies for the remaining stocks of these endangered fishes within Kenyan Marine ecosystems. Moreover, the LWR and condition information for these genus species in Kenya are clearly lacking from literature and data bases including those of FishBase. The results therefore provide invaluable information for the online FishBase database.


We express our gratitude to World Bank sponsored Kenya Coastal Development Project (KCDP) for funding the study through Kenya Marine and Fisheries Research Institute (KMFRI) Programmes; KMFRI Director for providing personnel and Institute facilities for the study’s undertaking; Fellow workmates (Joseph Kilonzo, Chanzera Kombe, Boaz Orembo, Eri Okinagwa Magara and Sammy Kadhengi) for assisting with sampling and analysis; Beach management unit (BMU) officials, fishery managers and local artisanal fishermen in Msambweni, Shimoni and Vanga for their excellent logistical work, connections and assistance during data collection. Thanks to all.

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