Fisheries catches and the carrying capacity of marine ecosystems in southern Brazil

Abstract

The carrying capacity of marine shelf ecosystems in southern Brazil for harvestable species is analyzed by (1) quantifying the amount of available primary production appropriated by fisheries catches, (2) evaluating the trend in the mean trophic level of fisheries, and (3) simulating the ecosystem effects of “fishing down the food web” in an intensively exploited shelf region. Fisheries utilize ca. 27 and 53% of total primary production in the southern and south-eastern shelf regions, respectively. Regional variation in the carrying capacity appropriated by fisheries results from differences in the primary production, catch volume and trophic transfer efficiencies. Overall, fisheries landings do not display a trend of decreasing trophic level with time due to the collapse of the sardine fishery and the recent increasing of offshore fishing for higher trophic level species, mainly tunas and sharks. However, the simulations show that fishing down the food web through fisheries that target small pelagic planktivorous fishes, while at first increasing catches in intensively exploited regions, has the potential of decreasing yields, by interrupting major energy pathways to exploited, high-trophic level species. The consequences of these results to the design of precautionary measures for future fishing policies are discussed.

Introduction

Recent assessments of the worldwide status of marine capture fisheries reveal alarming signs of human dominance and impact on the oceans. Fisheries alone appropriate ca. 8% of the total marine primary production and up to one-third of temperate continental shelf systems production (Pauly and Christensen, 1995). Over 60% of the most important fish stocks are either overexploited or at the limit of becoming overexploited by current fishing intensity (Garcia and Newton, 1997), and approximately 27 million t of non-target animals are discarded annually as “trash” fish (Alverson et al., 1994). Also, present exploitation patterns are resulting in a “fishing down marine food webs” phenomenon, from long-lived, high-trophic level piscivorous fish to short-lived, low-trophic level invertebrates and planktivorous pelagic fishes (Pauly et al., 1998).

In line with some of these global trends, marine capture fisheries of Brazil are in a state of crisis caused by the scarcity of resources, over-capitalization of fisheries activities and the lack of sound fisheries management policies. At the same time, there are major efforts to assess the potential production of fishery resources in the Exclusive Economic Zone triggered by the country’s ratification of the United Nations Convention on the Law of the Sea. Early assessments of the fisheries resources along the coast during the 1970s (Hempel, 1971; Neiva and Moura, 1977) indicated a potential total catch of over 1 million t per year, yet in fact, since the mid-1980s landings have stabilized around 700 000 t per year, and many of the traditional fish stocks have become either fully exploited or overexploited (Dias Neto, 1991a, Dias Neto, 1991b; IBAMA, 1994a, IBAMA, 1994b, IBAMA, 1994c; Reis et al., 1994; Cergole, 1995; Matsuura, 1995; Haimovici et al., 1997). This raises concerns as to whether the level of exploitation can be sustained without impairing the productivity and integrity of the marine ecosystems.

Brazil has an extensive coastline from 5°N to 34°S, including regions of tropical and subtropical climate. Matsuura (1995) divided the Brazilian coast in five regions with distinct environmental characteristics and types of fishing activities (Fig. 1). In the north, biological production is high as a result of the continental runoff from the Amazon river (Teixeira and Tundisi, 1967). The wide continental shelf and the rich benthic community favor the development of trawling activities in this region, mostly for shrimps and large catfishes. The northeast and east regions are oligotrophic due to the influence of tropical waters from the Brazil Current. Rocky bottoms and a mostly narrow continental shelf induced the development of hook-and-line fisheries for rockfishes, sharks and tunas. In the southeast, primary production is mainly driven by seasonal upwelling of nutrient-rich, cold subtropical waters pumped by alongshore winds and by cyclonic vortexes originated from the Brazil Current (Bakun and Parrish, 1990; Matsuura, 1995). The southern part of the Brazilian coast is under the influence of the subtropical convergence between the southward and northward flowing Brazil and Malvinas Currents. The confluence of water masses and the high volume of continental runoff provide physical and chemical conditions for high biological production on the shelf (Seeliger et al., 1997). Trawling is the main type of fishing activity in the south-eastern and southern regions, although the presence of highly abundant pelagic stocks, mainly sardine, in the southeast has also lead to the development of an important purse seine fishery especially since 1950.

The regions also differ in the type of fisheries production. While catches in the north, northeast and east regions are mainly from artisanal activities (Diegues, 1995), in the southern regions it is the industrial fisheries provide most of the landings, accounting for approximately half of the total Brazilian catches (IBAMA/IBGE, 1995). Historically, it was in the south and southeast that industrial fisheries were mostly developed through a series of government incentives, and this is where fisheries data are best documented.

This paper presents a comparative analysis of fisheries in the south-eastern and southern regions of Brazil which aims to assess the carrying capacity of the marine shelf ecosystems for harvestable species. Carrying capacity has been defined as the maximum size of a population or activity that could be indefinitely sustained without degrading the ecosystem’s future productivity or suitability for that use (Odum, 1997). In the oceans, carrying capacity is usually referred to as the upper limit of biomass of organisms that can be supported by a set of primary production and food web structure (Christensen and Pauly, 1998). Fisheries yield is directly related to the carrying capacity of marine ecosystems, since there is a maximum sustainable rate of fish production associated with the total fish biomass at the carrying capacity. Fisheries can directly affect the carrying capacity of marine ecosystems by altering the structure of food webs and changing their potential productivity. Ecosystem carrying capacity is analyzed in three ways. First by computing the total flux of energy, originated from primary producers, available to different trophic levels in the food web, and the total primary production required to sustain fisheries catches (Pauly and Christensen, 1995). The ratio of these two quantities provides a measure of the “appropriated carrying capacity” (sensu Rees, 1996) of ecosystems, i.e. the amount of the available energy in an ecosystem already appropriated by fisheries catches. Second, we present a diagnosis of fisheries for the “fishing down the food web” phenomenon using trophic level estimates and national and regional catch statistics. Third, the impact of fisheries on the structure of an exploited ecosystems is evaluated by simulating the effect of a “fishing down the food web” scenario in the southern shelf region, where traditional demersal fish stocks are overexploited and the prospects for increasing yield rely on exploiting abundant small pelagic forage fish.