Predicting changes to macrobenthic communities due to fishing disturbance in a shrimp fishing ground of the Mexican central Pacific

[Abstract] We test predicted short-term structural changes in the macroinvertebrate assemblages of a shrimp fishing ground in the tropical Mexican Pacific subject to low effort with seasonal closures. During two years, five cruises were carried out sampling seven sites and four depths (20, 40, 60 and 80 m) along 100 km of coastline. A series of community structure descriptors were used to determine the ecological effects of fishing disturbances: ABC curves, W-statistic, normalized species size distribution as biomass spectra, spatial segregation index, Shannon- Wiener diversity index, species richness and biomass. Theoretical predictions of the effects of fishing on these indices were tested by comparisons of samples collected from successive open and close fishing seasons. ABC curves show fishing has produced a state of chronic disturbance in the macroinvertebrate community. Shortterm effects could be masked by natural seasonal and interannual environmental changes. Results of the analyses of short-term effects are not in agreement with the fishing disturbance theories; only in 31% of the cases the results obtained of comparisons between successive cruises to test the different hypotheses about the effects of fishing were in agreement with theory. The temporal trend of increase of the biomass and the decline of species richness, could evidence interannual effects. The complexity of the sources of variability in a exploited community forces to focus on temporal and spatial gradients to understand the community structural processe

Interannual variability of the diversity and structure of ichthyoplankton assemblages in the central Mexican Pacific

[Abstract] We examined larval fish diversity and assemblage structure on the central Mexican Pacific (coast of Jalisco and Colima) using data from samplings carried out with a Bongo net at 12 stations during 27 months, from December 1995 through December 1998. A total of 132 taxa were recorded, and the dominant species were Bregmaceros bathymaster (90% of the total abundance), Dormitator latifrons (1.9%) and Harengula thrissina (0.8%). Only B. bathymaster, D. latifrons and Gobionellus sp. attained the 100 % of occurrence. The effects of the 1997-98 El Niño event in ichthyoplankton diversity were significant, however the prevalence of the normal seasonality could be observed. Diversity null models were used to determine structural changes in the assemblage due to El Niño effects; both species richness and evenness were highest during El Niño. The most parsimonious models of assemblage organization include the El Niño and seasonality as most significant environmental variability sources. The small-scale spatial variability expressed as the cross-shore gradient was not relevant. The dominant species group formed by B. bathymaster, D. latifrons, and Vinciguerria lucetia typify for similarity both the previous and El Niño period and the only change was the abundance difference among periods. The El Niño period was typified by the dominant species Bentosema panamense and H. thrissina, and by the rare species Euthynus lineatus, and species of the genus Lujtanus. The assemblage shows a similar organization in the different seasons, sharing the same dominant species group. Seasonality produces only changes in the abundances and relative frequencies of dominant species and different rare species are characteristic of the different seasons. The average taxonomical distinctness, that could be considered as a measure of functional diversity, was highly sensitive to the seasonal change of the assemblages independently of the El Niño; this index showed lowest values during tropical and transition periods characterized by warm and oligotrophic water

An assessment of sample processing methods for stable isotope analyses of coastal foodwebs

[Abstract] The relative abundance of the light and heavy forms of stable isotopes of Carbon and Nitrogen is commonly used in the research of marine food webs. Dehydration of samples is needed for C and N isotopic analysis, and in those organisms with inorganic carbonate structures acidification with hydrochloric acid is also needed for sample preparation. There is not a consensus about the methodology appropriate for sample processing. In the present study, the effects of different sample processing methods in the relative abundance of stable isotopes of C and N in the different trophic compartments of a coastal food web are evaluated. The final purpose is to define an adequate methodology to be used in the construction of whole food webs. The results obtained after acidification show a decrease in carbon values for sedimentary organic matter, suspended particulate organic matter, plankton and invertebrates with carbonated structures, and a variable response for nitrogen values. According to our results we recommend sampling acidification for carbon analysis in compartments in which variations of this element were observed. For nitrogen analysis acidification should not be done. Different treatments of dehydration studied provoked significant differences only in nitrogen values, thereby freeze-drying seems the most adequate method. Muscle tissue should be used in invertebrate with carbonated structures whenever body sizes make it possible

Hierarchical organization of spatial and temporal patterns of macrobenthic assemblages in the tropical Pacific continental shelf

[Abstract] 1. Nowadays, studies focused to test a well-defined set of a priori hypotheses about spatial and temporal organization of assemblages in marine communities are scarce. 2. This study presents the first fully-developed application of a parsimonious approach for model selection based on the Kullback-Leibler information theory using multivariate data to determine the best spatial and temporal models among a set of a priori alternative models of a soft bottom macroinvertebrate community in the tropical Pacific continental shelf (from 10 to 90 m deep). 3. Besides of the determination of the most parsimonious model, the Akaike´s information criterion allows the exploration of the scale-dependent patterns of community organization and to determine hierarchical relations among the gradients considered. 4. Depth the most conspicuous spatial gradient affecting macrofaunal assemblages, and the main discontinuity is located between 40 and 60 m. The degree of exposure is defined as a secondary gradient and it is only relevant in shallow waters. 5. Seasonal changes in the abundance along the bathymetric gradient of some portunid crabs and shrimps defined a process of seasonal recomposition, caused by a vertical shift of the community

Decomposing diversity patterns of a soft-bottom macroinvertebrate community in the tropical eastern Pacific

[Abstract] Diversity is one of the most frequently used attributes of ecological communities, and there is a long-standing discussion about their adequate use, calculus and interpretation. One of the most conspicuous and simple diversity estimators is species richness, but it has been criticized by recurrent pitfalls, which has biased comparisons and the testing of hypotheses. Otherwise, synthetic ecological indices of diversity are difficult to interpret due the underlying complex or cryptic concepts. Most of the studies in soft-bottom macroinvertebrate communities trying to estimate richness species, have actually quantified species diversity as species density, the number of species per unit area. In the present study, the diversity of a tropical Pacific soft bottom community is analytically decomposed using two null models to assess species richness and evenness, and the slope of species-area curves to estimate species density. A declining trend of species richness was observed along a 2-year period and could be related to the cumulative effects of interannual environmental variability and fishing disturbance. The species-area relationship could be considered a good indicator of spatial heterogeneity, specifically to illustrate the depth gradient. Local abundance was negatively correlated with evenness, showing that high-density communities are the result of increases in abundance of a few dominant species. Here we demonstrate that seasonality, depth and abundance could explain diversity in soft-bottom macroinvertebrate communities of coastal waters and propose a robust procedure to survey diversity as a part of monitoring programs of coastal management.Ministerio de Ciencia y Tecnología; REN2000-044

Effects of the 1997-1998 ENSO event in the eastern tropical Pacific coastal ecosystem

38th European Marine Biology Symposium, Aveiro (Portugal), 8-12 September 200

Plankton biomass and larval fish abundance prior to and during the El Niño period of 1997–1998 along the central Pacific coast of México

The temporal and spatial distributions of zooplankton biomass and larval fish recorded during 27 months (December 1995–December 1998) off the Pacific coast of central México are analyzed. A total of 316 samples were obtained by surface (from 40–68 to 0 m) oblique hauls at 12 sampling sites using a Bongo net. Two well-defined periods were observed: a pre-ENSO period (December 1995–march 1997) and an ENSO event (July 1997–September 1998) characterized by impoverishment of the pelagic habitat. The highest biomass concentrations occurred at coastal stations during the pre-ENSO period. During the El Nin˜o period no spatial patterns were found in coastal waters. The months with highest biomass were those in which the lowest sea surface temperature (SST) occurred (January–May), and this pattern was also observed during the ENSO period. A typical, although attenuated, seasonal environmental pattern with enhanced phytoplankton (diatoms and dinoflagellates) was prevalent during the El Nin˜o event in nearshore waters. During the El Niño period the phytoplankton was mainly small diatoms (microphytoplankton), while dinoflagellates were practically absent. The most parsimonious generalized linear models explaining spatial and temporal distribution of larval fish species included the ENSO index (MEI), upwelling index (UI) and distance to the coast. The environmental variability defined on an interannual time-scale by the ENSO event and the seasonal hydroclimatic pattern defined by the UI (intra-annualscale) controlled the ecosystem productivity patterns. The small-scale distribution patterns (defined by a cross-shore gradient) of plankton were related to the hydroclimatic seasonality and modulated by interannual anomaliesUniversidad de Guadalajara, Méxic

Habitat diversity and postlarval benthic decapod crustacean assemblages in shallow waters of the Ría de A Coruña (NW Spain)

38th European Marine Biology Symposium, Aveiro (Portugal), 8-12 September 200