Reviews in Fish Biology and Fisheries 12: 393–401, 2002. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.
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Changes in tropical fish assemblages associated with small-scale fisheries: a case study in the Pacific off central Mexico Elaine Espino-Barr1 , Arturo Ruiz-Luna2 & Arturo Garcia-Boa1 1 CRIP-Mzllo, Instituto Nacional de la Pesca. Playa Ventanas s/n. Manzanillo, Col. 28200, M´ exico (Phone: (52333)
23750; Fax: (52333) 23751; E-mail:
[email protected]); 2 CIAD-Mazatl´an. S´abalo-Cerritos y Estero del Yugo. Mazatl´an, Sin. 82010, M´exico (Phone: (5269) 880157; Fax: (5269) 880159; E-mail:
[email protected]) Accepted 12 February 2003
Contents Abstract Introduction Material and methods Results Fish species caught by artisanal catch Stability Diversity Discussion and conclusions References
page 393 393 395 396
397 400
Key words: assemblages, diversity, fishing impact, small-scale fisheries, stability Abstract Information from artisanal marine fisheries in Colima, Mexico is analyzed to find evidence of change in the composition of fish assemblages resulting from fishing pressure. Catch composition from up to 20 boats per day, during 2 to 6 days a month, were recorded from 1983 to 1998. Stability of the species composition was analyzed using rank correlation methods, and measures of diversity (specific richness, Shannon-Wiener index, evenness) were estimated from data. The resulting time-series were analyzed to detect trends. No significant differences were found in the number of species caught by month nor were significant trends in diversity detected. The composition of the fish assemblages is dynamic. The presence of persistent and resilient species that defined the values obtained of diversity and evenness was detected, but did not determine the structure of the community, which is apparently defined by other factors. The changes in composition of the catch probably are a consequence of environmental fluctuations and random events, and not a direct consequence of fishing pressures on the fish community.
Introduction Fishing, recognized as the most extensive human exploitative activity in aquatic systems, can have a negative effect on local fish populations and their habitat, driving them to critical ecological stages and even to local extinction. The response of fish stocks to management or control strategies is mostly unknown (Buckworth, 1998), and significant uncertainty about the condition of target species populations remains,
despite advances in gathering and processing fishery information. However, there is enough evidence that fishing pressure is changing some of the attributes and characteristics of fish communities and their environment. Jennings and Kaiser (1998) characterized the main changes as direct or indirect alterations of the habitat, changes in the structure of populations and communities (e.g. diversity, size structure, life history traits), and changes in the trophic interactions (e.g.
394 removal of predators, substitution of species). Other effects on aquatic communities come from activities such as inland agriculture and industry by increasing pollution, nutrient runoff, and eutrophication (Symes, 2001). The aquatic communities are also affected by harbor activities, infrastructure development, changes in the hydrologic cycle, and human activities that modify the landscape and produce further effects on the aquatic biota (Ruiz and Berlanga, 1999). These alterations have diverse consequences on the ecosystem and affect the diversity and the equilibrium among species. One of the central themes in the changes taking place in world fisheries emphasizes the present level of exploitation of the resources. It is commonly thought that most of the main fish stocks have arrived at their limit of production and consequently a significant increase in global production is highly improbable (FAO, 1999). For some fisheries experiencing exploitation levels over or near their maximum, changes in their communities, and even in related species such as birds, have been reported (Goñi, 1998; van Ginkel, 2001). Some of these changes have been explained by analyzing the structure of the population or the composition of species taken as catch (Brander, 1988; Koslow et al., 1988; Harris and Poiner, 1991; Goñi, 1998), and the variation of the catch as a result of changes in effort (Russ and Alcala, 1989; Greenstreet et al., 1999). For tropical waters, Pauly and Murphy (1982) integrated a series of studies, most of them related to Asiatic fisheries, where exploitation of multispecies stocks were analyzed. In tropical latitudes, reefs have received special attention because of their vulnerability to the effects of fishing (Koslow et al., 1988; Russ and Alcala, 1989). In spite of this, Harris and Poiner (1991) indicated that most of the studies done in tropical zones, aimed at detecting tendencies of change in the composition of species of commercial fishing, rarely cover more than 5 years of observations. This situation is the result of the limited importance that tropical countries give to the recording of information of artisanal fishing, and because of the complexity and high diversity of species and of fishing systems. It is not the same for European inshore fisheries, where the problem is not the lack of information, but they do have problems with the high diversity in fishery systems and the variation in the institutional arrangements for management (Symes and Phillipson, 2001).
In Mexico, as in many other countries, the importance of artisanal fishing is now being evaluated because it is an important source of employment and supplies fresh products for direct human consumption (SEMARNAP, 1996; SAGARPA, 2001). More attention has been given to these fisheries by the federal and local authorities, and the collection of catch data is, at present, more reliable. These data permit studies on the communities of fish in tropical zones to go beyond simple analysis of global captures and systematic comparison of listings. Improvements in the recording of catch data (e.g. length structure, age, and growth), and greater precision in the capture records (composition and abundance) help determine trends in the composition of commercially caught fish, and reflect the characteristics of the fish community. We have used information collected for more than 15 years of a Mexican artisanal fishery in the Pacific to evaluate any changes the fishing has had during this period. Composition of the catch, reduction in the average length of the target species, and variation in the length structure have been reviewed (Cruz-Romero et al., 1989; Espino-Barr et al., 2001). However, the evaluation of processes at the population level, although important, only partially reveals the state of the community. The present study concentrates on the analysis of diverse aspects that define the structure of the community, such as specific richness, diversity, and the proportion and persistence of species in the catch, as a measure of the stability in the community, understood as the similarity in the composition of captured species in repeated samplings (Margalef, 1980). The mechanisms that regulate the abundance and diversity of species in a community include the temporal scale, spatial heterogeneity, environmental stability, and some ecological and biological processes, such as productivity levels, competition, and predation, though lately it has been proved that these latter two have a lesser role in the structure of the assemblages than thought previously (Gaertner et al., 1999). Thus a disturbed community becomes unstable and goes through a process of readjustment with changes in the biological variables of the species, which suggests a tendency to regain stability through a deterministic mechanism or an equilibrium process such as happens in constant habitats or under the influence of predictable fluctuations (Grossman, 1982; Boulton et al., 1992). With this perspective, an attempt was made to evaluate the present state of the artisanal
395 fishery in Colima to assess the course of these changes, and more specifically to determine to what extent fishing is affecting the community of fish. The results are proposed as a supporting element for local fisheries management. They are also important to promote the regional conservation of marine diversity.
Material and methods The information was recorded from catch made by the artisanal fishermen in the coastal waters off Colima, Mexico. This coast, in the central region of the Mexican Pacific, is about 160 km long. The fishing area is limited by the 200 fathom (around 370 m) deep isobath that extends 15 to 30 kilometers off the coast. This coast is characterized by a succession of rocky and sandy shores, and it is influenced by Rios Marabasco, Coahuayana, and Armería (Figure 1). Artisanal fishing operates throughout the shelf, but most of the fish products are unloaded for marketing at four locations, namely Miramar, Manzanillo, Boca de Pascuales, and Boca de Apiza (Figure 1). The artisanal fishery consists of approximately 1000 small boats, working roughly 250 days a year, and catching an average of 25 kg·day−1. Since November 1982, a monthly sampling program has been done to obtain information on the catch per boat, especially fish composition (total weight and by species) and other characteristics, relevant to fishery studies. The time-series used here, from January 1983 to December 1998, consists of 123 months. The study was temporarily interrupted from May 1993 to July 1996 because of institutional personnel shortage. Fishing is mainly by hook and line, and to some extent with various net gear. Therefore fish representative of diverse habitats are found in the catch. During the study period, catch data were recorded from 5 to 20 boats per day and for 2 to 6 days per month, depending on the fishing trips, prices of the products, and sea and weather conditions. The entire catch was taxonomically identified to species level based on the reference collection found in the Centro Regional de Investigación Pesquera, Manzanillo, Colima. Although the data base included information on the fishing zone, depth, type of gear, landing sites, and other information appropriate to fisheries studies, only the data for the monthly catch (kg·month−1) by species was used in the analyses. To make comparisons among monthly catch, data of catch by species
were normalized as the proportion of the total catch by month (%). To determine the changes in the number of species caught by month, Spearman rank correlation analysis was used, assigning ranks to dates in correspondence to the number of species identified each month during the study. The Spearman coefficient values rank from −1.0 to 1.0. The boundaries are for extreme cases where there is either none or total coincidence in the order followed by both variables (Zar, 1996). After that, species whose presence was rare or occasional were eliminated from the matrix for a second analysis taking into account only permanent or frequent species that make up the main structure to the community. The stability of the temporal fish assemblages was assessed using Kendall’s coefficient of concordance (W) analysis, ordering the species by annual catch proportions, and obtaining their importance intervals (Rij). When the importance levels maintain the same order in the temporal series, there is maximum concordance (W = 1.0), which means high stability in the fish assemblages. A variance analysis was made using a Kruskal-Wallis test to test changes or variations among samples, where nk observations obtained from k samples are combined in a series of n size in growing order of magnitude and their values are replaced by a rank, with the minimum value of 1 used for the smaller (Zar, 1996). We used another technique to determine whether the composition of captures was stable during the period. Tests with the contingency table with X2 allowed us to determine differences among relative abundances and to determine if they originated from independent events. Columns (years) were compared independently from the rows (species). Finally, to detect changes in species diversity associated with changes in fish composition, the ShannonWiener function, as described by Margalef (1980), was calculated with base e log, considering simultaneously the number of species and their abundance in the community. Monthly components were also calculated; specific richness (d = S − 1/log(N)) and evenness (e = H /log S). In both formulae S represents the number of species; N is equal to the number of individuals and H indicates the value of the index of diversity. The indexes were calculated monthly to produce time-series, which were then graphed to analyze and observe changes over time. A moving average to eliminate noise by extreme data was included. A linear model was fitted to the time series of diversity by means of linear regression analysis,
396
Figure 1. Boundaries of Colima State and 200 fathom depth limits.
which included the examination of residual errors and their significance using an analysis of variance (ANOVA) of the regression model. Results Fish species caught by artisanal catch During the study period, 39,433 individuals and 140 fish species captured by the artisanal fleet were sampled. Species from different habitats, trophic levels, and ages were found. The 140 identified species were part of 51 families, with most belonging to Carangidae, Lutjanidae, and Haemulidae, i.e. 12, 9, and 8 species respectively. The most abundant species were red snapper “huachinango” (Lutjanus peru) and the spotted rose snapper “lunarejo” (L. guttatus). An average of 25 species were identified per month, with a maximum of 63 species (March 1992) and a minimum of 1 (January 1985). Fitting a linear regression model to the number of species by month showed a negative tendency (b = −0.05 species·month−1) that is not
statistically different from 0 (Figure 2). Although this result implies a reduction of almost 10 species in the average monthly figures during the period of study, the information gap (June 1993–May 1996) and the fluctuations observed in the monthly species number may bias the trend. Data displayed large temporal fluctuations probably associated with cyclic phenomena, which are not verifiable considering the size of the time-series, but strongly influence results for the linear fitting. Stability The Spearman coefficient rank correlation test for 123 values obtained for monthly species number gave a result of rs = −0.173 (P > 0.05), thus rejecting the null hypothesis of dependence between variables. Therefore, it is probable that the composition observed in any specific time will have future effects on the community structure. Figure 2 shows data that the largest number of species were consistently present from 1986 to 1987 and from 1992 to 1993. These periods coincide with El Niños. Although this
397
Figure 2. Number of species recorded per month of sampling, and the calculated line and equation.
phenomenon was a strong event in 1997 and 1998, the number of species, even if high, is not as large as during other El Niños. For further analysis, the discard of rare or occasional species limited the data matrix to the 30 most abundant species, which generally amounted to 85% of the annual total catch in Colima (Table 1). Individually, the figures by species ranged from 0.4% to 17% of the total catch recorded during the study. The annual proportion per species was used to calculate the coefficient of concordance (W = 0.8), significant with a test of X2 , r = 354 (P < 0.05). This indicates that there was association among the years. Based on this information, the Kruskal-Wallis nonparametric variance analysis was done, obtaining a statistical value H = −4422 and X2 0.05,14 = 23.7 (P < 0.05). According to this result, the differences of composition among years were not statistically significant. This result was reinforced with that obtained by the contingency table analysis, where the sum of elements approached 1,594 (P < 0.05), indicating dependence among the variables. Diversity Specific richness (d) for fish caught in Colima ranged from 0.4 to 20.3 throughout the study period (Figure 3a), with minimums in 1984, 1985, 1989, and 1990, and with a reduction to the end of the time series (r = −0.43, P < 0.001). Similar patterns for the values of the evenness index (e) and diversity (H ) were found. These parameters also displayed important variations associated mainly with seasonal changes (Figure 3b,
c). The average value for the index of evenness was 0.83 bits per individual, whereas the maximum values remained between 1.3 and 1.4 bits per individual. These maximums were present only during the first six years of the study. From 1988 onwards, the maximum values observed are under 1.2 bits per individual, following a negative trend. After smoothing data with the moving average technique, regression analysis indicated that the slope (b = −0.003) was not different from zero (r = 0.61, P < 0.001). This implies a slight reduction by month, around 0.6 bits per individual (from 1.4 to 0.8) during the period of study, which represents almost 40% of the maximum observed value. Discussion and conclusions From our results, the marine artisanal fishery of Colima uses a large number of fish species, whose community structure, seen as the relation between fish composition and their abundance, seems to be more dynamic than stable in the short term. The number of species caught monthly by the artisanal fishery showed large variations that probably are determined by environmental conditions. The higher numbers were always associated with dates when El Niños were present, increasing the water temperature. However, this relationship cannot be proved, as no temperature data are available to be associated with the catch. In general, there was a reduction of the average number of species caught over time, statistically not significant at the monthly level, but enough
398
Figure 3. Time series of ecological indexes for the fish assemblages caught by the small scale fishery of Colima, Mexico, and the calculated line and equation.
399 Table 1. Common and scientific names for the 30 most abundant species in the artisanal marine fisheries of Colima, Mexico. The species were ranked in accordance to the proportion amounted to the total catch obtained from 1983 to 1998 Id
Common name
Scientific name
Proportion (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Huachinango Lunarejo Dorado Sierra Colmill´on Barrilete Alaz´an Jurel Listoncillo Cocinero Cabrilla Ojo de perra Puerco Baqueta Bacoco Gallo Tecomate Viejita Ronco chano Medregal Pintillo Sand´ıa Lora Cuatete Rasposa Robalo Zulema Palometa Curvina Palmilla
Lutjanus peru Lutjanus guttatus Coryphaena hippurus Scomberomorus sierra Lutjanus jordani Euthynnus lineatus Lutjanus argentiventris Caranx caninus Lutjanus colorado Caranx caballus Epinephelus labriformis Caranx sexfasciatus Sufflamen verres Epinephelus acanthistius Anisotremus interruptus Nematistius pectoralis Hoplopagrus guntheri Paranthias colonus Haemulon flaviguttatum Seriola rivoliana Epinephelus analogus Lutjanus inermis Scarus perrico Arius seemanni Haemulon maculicauda Centropomus nigrescens Sectator oscyurus Trachinotus paitensis Ophioscion strabo Trachinotus rhodopus
17.0 10.8 10.2 5.9 5.1 4.4 4.3 4.1 3.7 2.4 2.2 2.2 1.6 1.4 1.4 1.3 1.2 1.0 0.9 0.7 0.7 0.7 0.7 0.6 0.6 0.6 0.6 0.5 0.5 0.4
to weight on the final results concerning stability. When the number of species by month was analyzed, periodic fluctuations were most likely determined by previous conditions. After 14 years, the average of species caught per month has been reduced by almost 10 species. However, during the study period, the most abundant species were always caught in similar proportions. Others appear to be resilient species, probably responding to environmental changes. In either case, the persistent and resilient species seem to define the structure of the fish assemblages assessed in this study. However, it is probable that there is no single determining factor but trends affected by multiple causes.
Information derived from commercial fishing has been used to propose different indicators of the state of captured fish populations. Catch per unit effort, the average length in the catch, and volumes extracted are primary indicators of the state of a fishery, but the information derived from these sources are also valuable to analyze the fish community or taxocen as a whole. Considering that, and supposing a constant activity of fishermen, the information derived from the artisanal fishery may provide indices of change in the fish communities. Simplistically, the term community applies to assemblages or groups of species that interact among themselves and with their environment (Beals, 1960; Krebs, 1985; Dunson and Travist, 1991). The degree of interaction among the species and their environment causes diverse temporal and spatial distribution patterns that are relatively stable in time, subject to long term processes of extinction and speciation, which can be modified by natural events or by human activities such as fishing. As a reflection of the natural community processes, the temporal and spatial patterns of distribution of any species, when fishing effort is stable, will be maintained if variations in the catch are only modified by seasonal, short-term changes. For unstable environments, the patterns of distribution may be modified by cyclic environmental phenomena, as seems to occur with the present data, but these patterns could be restored if the community structure is stable. This structure has been found to be influenced by the physical environment more than species interactions (Madrid and Sanchez, 1997; Gaertner et al., 1999). The structure of the community responds to processes and very complex patterns that need to be analyzed in a known hierarchical framework and at different time scales. For this reason, although the series analyzed in this study is relatively short to emphatically define changes in the structure of the fish community, it is reasonable to make some inferences based on short time changes, some of which may be caused by human actions on the environment. Fish consumption in Mexico has begun to diversify only in the last 20 years and the number of species caught monthly seems to be declining. Though it may be a temporary effect associated with periodic fluctuations, which could not be evaluated because of the size and characteristic of the time-series, it should be emphasized that the majority of the species number data recorded since 1996 were lower than the earlier average. Similar effects, i.e. negative trends, have
400 been observed for nontarget species (Russ and Alcala, 1989; Harris and Poiner, 1991) and for commercial species (Koslow et al., 1988), although related more to the abundance than to the diversity. In every case, the result has been associated with direct and indirect effects of commercial fishing. The group of species in a community is not static, but there is a relationship with composition of the catch, and changes may be detected by studying fish capture data. Particularly for Colima, the artisanal fishery acts on a large group of species and probably because of the use of the principal fishing gear (line and hook), the best represented families (Carangidae, Lutjanidae, and Haemulidae) are from the upper trophic levels. A reduced number of species typically constitutes the majority of the catch, showing important changes during the study, even in the proportion that those species represent. Similar results for some areas of the Mexican Pacific have been found by Ruiz and Madrid (1997) who reported equivalent values of diversity, with high proportions of the catch formed by a reduced number of species. The analyses done on monthly species numbers indicated that the composition of the catch could be defined by previous events. When only the 30 most important species were taken into account, it was observed that in spite of the variations in the reported volumes for these species, there was no significant difference among years and the proportion for these species remained stable. Such kind of species are always present in every community, and they are considered as the persistent or resilient component of the communities (Law and Blackford, 1992). Such species define the values obtained of diversity and evenness, but do not determine the structure of the community, which is apparently defined by other factors. In agreement with some authors, the environmental factors have more influence in the maintenance of the community structure than interspecies relationships, and research should be directed at finding which one can be used as an indicator (Lluch-Belda et al., 1991; Laevastu, 1995; Santana-Hernández et al., 1996). Therefore, it would seem possible to explain the increase in the number of species and in diversity of the fish assemblage as consequence of oceanographic events such as El Niño observed during 1982–1983 and 1986–1987. Similar relationships have been proposed for changes in the fish composition along the coast of Michoacan, Mexico (Madrid and Sanchez, 1997). However, during the recent El Niño (1997–
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