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5. AN ASSESSMENT OF THE BRAZILIAN SPINY LOBSTER, P. ARGUS, FISHERY


Nelson M. Ehrhardt[16] and Carlos Artur Sobreira Rocha[17]

Introduction

The Brazilian spiny lobster, P. argus, inhabits shallower areas of the continental shelf between the States of Espírito Santo and Amapá in northeastern Brazil where it supports one of the largest spiny lobster fisheries in the Western Central Atlantic Ocean. It is a fishery of the greatest economic and social importance in Brazil and to the State of Ceará in particular.

From the late 1960s and until the mid-1980s the fishery consisted of a mechanized industrial fleet that concentrated most of its trapping effort on fishing grounds off the coast of the State of Ceará. Since the late 1980s, the industrial fleet has been replaced by semi-industrial and artisanal fleets in response to an increased cost of fishing derived from significant increases in fuel prices, the opportunity to re-introduce sails as a cheaper means of propulsion in smaller vessels, and to a general deterioration of the Brazilian economy that generated large unemployment resulting in a significant labour migration to the open access spiny lobster fishery. This transition represents an "artesanalization" process of the industrial fishery that today employs over 15 000 fishers using gillnets as the main gear to catch lobsters. In spite of the great increase in fishing effort and the damaging effects on spiny lobster populations created by the legalization of the use of gillnets in the 1990s, landings in the last two decades have remained at an average of about 5 000 tonnes per year with a large inter annual variance. The fishery has a total annual export value of about US$60 million and it continues to be one the most significant export commodities in the State of Ceará.

Generally, there is an excess of fishing capacity in the Brazilian lobster fishery that has created increased competition among fishers for a limited spiny lobster production, and as a result, the average per capita income of fishers has decreased significantly in the last decade. The excess in fishing capacity is also reflected in the increased percentage of lobsters below the legal size limit landed by the artisanal fleets using gillnets and scuba diving. This characterization of the fishery has complicated the monitoring and control of the fishery management strategies implemented by the government to manage this fishery in a sustainable way.

In the workshop held in Mérida, Yucatán, México, in 2000, the stock assessment database for the Brazilian spiny lobster fishery was revised and integrated for the period 1974-1993. The Marine Biology Laboratory (LABOMAR) at the Federal University of Ceará in Fortaleza, Brazil generated most of the information contained in that database. For this Annex, the database was updated and expanded to include all the historic information available at LABOMAR to include the period 1970-1997. The biological data for the years 1998 and 1999 available at LABOMAR were statistically insufficient to estimate catch-at-length (age) in overall landings. This was due to small biological sample sizes and the highly stratified nature of the samples, which represented only some of the artisanal landings in the State of Ceará, hence, expansion factors could not be properly developed to represent total landings by size. Consequently, these data were not included in the updated database and analyses.

Starting in 1999, the (federal) Brazilian Institute for the Environment (IBAMA) became the institution in charge of collecting the information necessary for spiny lobster stock assessment and management in northeastern Brazil. Future stock assessment work will need to couple the two databases, and this task will require a thorough revision of the effects of differences in experimental sampling designs on the quality of the overall stock assessment database. Therefore, this Annex represents the last analysis of the fishery that will use the LABOMAR database as a unique source of information. The results presented in this Annex were obtained following the same age-structured stock assessment methodologies already detailed in the Report of the FAO Workshop held in Mérida, Yucatán, México, in 2000.

Stock Assessment Results

The stock assessment results contained in this report are based on a least squares algorithm implemented to calibrate a Sequential Population Analysis (SPA) process. Standardized seasonal catch per unit of effort was used as a calibration index and the objective function that was minimized by the algorithm was the sum over years and ages of the squared difference between the observed and expected catch per unit of effort at age in each year. The algorithm allowed the catchability coefficient to be estimated annually and the expected catch per unit of effort at age was estimated as the product of the annual catchability coefficient and the average abundance at age estimated by the SPA procedure. Further details on the stock assessment algorithm are found in the 2000 Mérida Workshop Report (FAO Fisheries Report No. 643).

Estimates of the seasonal abundance in numbers (NSPA) of the Brazilian spiny lobster stock are given for females and males in Figs 1 and 2, respectively. For comparison purposes the results of spiny lobster abundance in numbers estimated at the 1997 Belize Spiny Lobster Workshop (FAO Fisheries Report No. 619) by a tuned length cohort analysis (NTLCA) procedure (Ehrhardt and Legault, 1996) are also included in the figures. The two very distinct stock assessment methods appear to generate very similar general trends in abundance with both male and female stocks showing important increasing and decreasing trends throughout the history of the fishery. A slightly larger abundance is observed in the female stock during the period 1988-1994.

Five age groups (2 to 7) contribute to the landings with a significant preponderance of ages 2 and 3. This condition is found also in the abundance-at-age estimated by SPA, consequently, the historic trends in annual stock abundance observed in Figs 1 and 2 cannot be simply explained by fishing mortality but fundamentally as a result of recruitment variability.

Using the estimates of annual abundance of age-2 individuals as recruitment (R) and selecting the abundance of ages 3 and older in the assessments as the parent stock (P), it was possible to analyse the nature of the trends in recruitment, the density-dependence of recruitment to parent stock abundance, and to correlate recruitment success (ln(R/P)) with wind intensity. Intensity of the wind is an environmental variable that is thought to be responsible for significant changes affecting the larval spiny lobster retention mechanisms off northeastern Brazil.

Figure 1. Female spiny lobster abundance in numbers estimated by tuned length cohort analysis (NTLCA) and by age-based sequential population analysis (NSPA)

Figure 2. Male spiny lobster abundance in numbers estimated by tuned length cohort analysis (NTLCA) and by age-based sequential population analysis (NSPA)

Recruitment trends of males and females (Fig. 3) indicate the existence of large inter-annual variation following very approximate trends between the two sexes. Some of the most conspicuous decreasing trends in recruitment appear to be related to major El Niño-Southern Oscillation (ENSO) events in the Pacific Ocean. It is well known that extensive droughts occur throughout northeastern Brazil during ENSO years due to the relaxation of the easterly winds flowing across the South American Continent during those years. Conspicuous decreases in recruitment are observed in Fig. 3 during 1973-1975 following the significant 1972 ENSO, in 1983-1985 following the strong 1982 ENSO, and finally in 1995-1997 as a consequence of the persistent but mild 1991-1993 ENSO years. Although it is a speculation, the impact of the very strong 1997-1998 ENSO on recruitment abundance should be observed in the future assessments corresponding to the period 2000-2002. This possibility may be highly likely, given that spiny lobster landings in the State of Ceará decreased from a low 3 372.2 tonnes in 1997 to 2 833.3 tonnes in 2001, while the fishing fleet increased from about 2 000 to 2 412 vessels.

Figure 3. Male and female spiny lobster recruitment abundance

The probable ENSO effect on recruitment is shown in Fig. 4 where the anomalies (observation minus mean divided standard deviation) of the wind intensity and of the 2-year delayed recruitment rate (R/P) are observed. In the figure, higher recruitment rates are always associated with lower wind intensity and vice versa. The lower wind intensity anomalies correspond to non-ENSO years, and in fact (although not shown in this report) we found a significantly high correlation between the reported wind speed anomaly and the ENSO anomaly in area 1 + 2 of the Pacific Ocean.

Figure 4. Wind intensity and recruitment rates of spiny lobster in Northeastern Brazil

The character of the density dependence of recruitment on parent stock is shown in Fig. 5. In the figure the trend can be expressed by a linear equation such a ln(R/P) = -0.000000051* (2-Year Delayed Spawning Stock Abundance) + 1.173827217 which has a coefficient of determination of R2 = 0.596.

Figure 5. Density dependent relationship between recruitment success and parent stock abundance of the spiny lobster in northeastern Brazil

Figure 6. Fishing mortality rates for males and females spiny lobsters

Most of the variability about the regression line is due to ENSO-related wind effects in northeastern Brazil. We developed a multiple regression equation that accounts for about 94 percent of the total variance in recruitment success when the independent variables are parent stock abundance delayed two years and wind speed also delayed two years. The results of the more extensive study on recruitment dynamics of spiny lobsters in Brazil have been submitted for publication in the Fishery Bulletin.

The results shown above are indicative that the spiny lobster fishery in Brazil is fundamentally a recruitment driven fishery. Under the historic increasing trend in fishing effort, long-range landings have remained stable (no trend) with significant variance. The resulting effect is that fishing mortality rates have varied greatly as a result of population abundance changes. This is observed in Fig. 6 where the seasonal fishing mortality rates estimated for males and females are presented.

Fishing mortality rates are slightly higher among males than females and in general it is observed that the fishing mortality rates in both sexes are much higher than the natural mortality rate adopted in the assessments (M = 0.35). This may be interpreted as a clear indication of an excess of exploitation being exerted on the stock throughout the entire period of analysis. The considerable number of new vessels that annually enter the fishery exacerbates this condition; while the intensification of the legal use of non selective gillnets by artisanal fishers and the increase in the unregulated use of scuba and free diving in the fishery are two factors that add up to the high levels of fishing mortality observed in the fishery. These technological changes represent a large change in the fishing power capacity of the fleets as well as in the exploitation patterns inflicted on the stock. Historically, fishing effort increased from 26.5 million trap-days fishing in 1974 to 36.9 million trap-days fishing in 1984 and to 56.8 million trap-days fishing in 1994. Hence fishing effort more than doubled in 20 years with the consonant that expansion of the fishing operations to shallower areas resulted in an increased utilization of younger individuals. Consequently, the stock assessment results show a very significant change in the exploitation pattern observed during the last 3 decades (Fig. 7). These results are indicative of the need to establish stricter controls on fishing capacity and more importantly on the modality of fishing.

Figure 7. Changes in exploitation patterns observed in the spiny lobster fishery

Conclusions

The Brazilian spiny lobster fishery is undergoing significant changes in exploitation as a result of large shifts in the quantity and quality of the fishing effort resulting from incorporating large numbers of sailboats (about 1 300 in the last decade) and smaller motorized crafts (over 700 in the last decade) that typically use non selective gear for catching spiny lobsters (i.e. gillnets and diving) as their main gear types. These changes have resulted in a considerable shift of the fishing intensity from deeper to shallower coastal fishing grounds, and as a consequence, the smaller yet not mature spiny lobsters are currently subjected to high levels of exploitation.

The significance of the environmental signals on Brazilian spiny lobster recruitment is large and the overall stock abundance trends are fundamentally governed by recruitment changes. Landings in this fishery are the result of a high fishing capacity exerted on a variable stock abundance that results in very high fishing mortality rates during those seasons of low stock abundance and vice versa. As the process of increasing fishing capacity continues, the severity of the impact on the stock may be much larger as the density dependence of recruitment on parental stock may increase the risk of a major collapse is the parental stock is significantly depleted. Depletion of the parental stock appears to be a natural cause in this fishery as the number of juveniles that are incorporated to the landings continue to increase as fishing effort increases.

Artesanalization of the industrial fleets has created many problems in the monitoring and control of the fishing capacity and of the exploitation pattern that are being exerted on the stock. Controls on entry to the fishery and on the gear used by fishers will become more difficult to be politically implemented by the authorities as the number of fishers participating in this fishery increase and the economics of subsistance finally characterize this fishery.

References

Ehrhardt, N.M. & Legault, C.M. 1996. Crustacean stock assessment techniques incorporating uncertainty. In: Report of the WECAFC Ad Hoc Shrimp and Groundfish Working Group of the Guianas-Brazil Continental Shelf and CFRAMP Shrimp and Groundfish Subproject Specification Workshop. Port of Spain, Trinidad and Tobago. 8-12 January 1996. FAO Fisheries Report No. 544, Supplement, pp. 111-131.


[16] Division of Marine Biology and Fisheries, Rosenstiel School of Marine and Atmospheric Science, University of Miami.
[17] Marine Biology Laboratory, Federal University of Ceará, Fortaleza, Ceará, Brazil.

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