Recruitment of sea breams (Sparidae) and other commercially important species in the Algarve (Southern Portugal) - 2000-2002. DG XIV, Ref. 99/061

Objectives and outline of the project:

In this study, we proposed to study:

Different habitats were studied with the use of a variety of appropriate sampling gears. The sampling strategy was based on an existing Geographic Information System (GIS) application incorporating a variety of databases, allowing the identification of different habitats (depth, sediment type, vegetation) and subsequent stratified sampling. Mortality was studied by analysis of the evolution of size distributions over time. Fish movement and recruitment from the Ria Formosa to the fishing grounds were studied by tagging of fish greater than 8 to 10 cm in length.

This work was build on and complement the pioneer study of Monteiro (1989) on the fish community of the Ria. This contributed to the understanding of the recruitment process in demersal species, particularly in terms of habitat requirements for different species, times of settlement/recruitment to the Ria, and the relationship between environmental variables and settlement/recruitment. It also allowed an evaluation of changes in species diversity and relative abundance over a 13 to 15 year period, since no systematic sampling of the fish community of the Ria Formosa has been carried out since Monteiro (1989). From the perspective of management, this study contributed with valuable information on critical coastal habitats.

Summary for non-specialists

The Ria Formosa is lagoon system located on the south coast of Portugal that consists of barrier islands, creeks, channels and salt marsh. It is a highly productive system that provides ideal conditions for juvenile fishes. Previous studies have show that the Ria Formosa is an important nursery for many commercially important species. Our main objective was an in depth investigation of the juvenile phase of the fishes in the Ria. Specific objections included the study of the species composition and abundance in different parts and habitats of the Ria, the determination of the time of settlement/recruitment of the most important species, the study of habitat use and preferences, the study of some of the factors affecting distribution and abundance, the study of movements and dispersal of some of the most important sea bream species and the comparison of the present day (2001-2002) distributions, abundance and species composition with those of an earlier period (1981-1986).

Because the Ria Formosa is characterised by a variety of different habitats, from deep water channels to shallow creeks in the salt marsh, it was necessary to test and employ different sampling methods. Thus, for the secondary channels the main sampling method was the beach seine. This is an encircling gear made of fine mesh and with a float line and a lead line that is effective in fishing the entire water column. For the deeper channels a beam trawl 2.6 m wide and 0.45 m high was used. The shallow water areas such as creeks were sampled with a Riley push net 1.5 m wide and 0.5 m high. A throw net with sides of 1 m and 0.6 m high was used to sample specific shallow water habitats. In addition, a lift net was tested for the same purposes as the throw net but proved to be ineffective and was not used.

In order to characterise the ichthyofauna of different habitats and to study the distribution, abundance and time of settlement, monthly sampling took place with all the gears. The 25 m beach seine was used to sample 37 different sites in the western part of the Ria Formosa on a monthly basis. The sites were selected after the entire area had been classified into 8 distinct zones based on a series of characteristics. Sampling was at low tide and the fish were sorted, identified, measured and weighed in the laboratory. Sampling with the beam trawl took place on a monthly basis at 12 sites (channels), with 300 m tows. The Riley push net was used at 6 sampling sites representative of different habitats, while the throw net was used in one site where three different bottom types were sampled intensively (20 samples per habitat type per month). In order to study changes due to the state of the tide and day/night differences, the beam trawl was used to carry out sampling over a 24 hour period on a seasonal basis.

For the comparative study, 4 sampling locations from an earlier (1981-1986) study were sampled on a monthly basis in 2001-2002 using exactly the same methodology. A 50 m beach seine was used at low tide, with 3 successive sets in the same location.

Movements of juveniles were studied by tagging 4315 fish, mainly commercially important sea breams and European sea bass with Floy brand T-tags. Survival of tagged fish was evaluated in a three month laboratory study involving 240 sea breams of the species Diplodus vulgaris.

Throughout the field work environmental parameters (temperature, salinity and oxygen) were recorded and precise geographical co-ordinates of the sampling points were obtained using GPS (Geographic Positioning System) technology. A data base was created and the data incorporated into a GIS (Geographic Information System) for analysis and mapping. The data were analysed using a variety of multivariate statistical methods as well as simple and descriptive statistics.

The results of our study confirm the importance of the Ria Formosa as a nursery for juveniles of a large number of species. We recorded a total of 112 species, a significant increase compared to the total number reported by Monteiro (1989) who sampled intensively at 8 locations with a 50 m beach seine from 1981-1986 and identified 67 in total. The sampling with the 25 m beach seine alone accounted for 87 species belonging to 33 families and 52 genera. Many species have not been previously reported from the Ria Formosa or even the south of Portugal. Three types of species were identified: resident (found all year and spending their entire life cycle within the Ria), migratory (spending part of their life cycle in the Ria) and occasional (rare species). Resident species included some of the most abundant species, including Atherina spp., that dominates in numbers and biomass, various species of gobies and mullets. The migratory category includes most of the commercially important species such as sea breams, sea basses and the sardine. These are species that largely use the Ria as a nursery, entering as small juveniles and leaving the Ria before they complete their first year of life. Occasional species are those found in small numbers and on an irregular basis, especially during the warmer months. In general, a few species dominated the catches of all the gears. In the case of the 25 m beach seine for example, 16 species accounted for 95% of the total catch in numbers, with Atherina spp. alone representing 58.3%.

Diversity was lowest in the winter, increasing to a maximum in the summer and autumn. The increase in diversity was largely due to the migratory category species, the majority of which enter the Ria as small juveniles in the late winter/spring. The analysis of the monthly length frequency distributions, by gear and for all gears combined allowed the determination of the recruitment/settlement period for the most important species. Thus, while the resident species are characterised by largely summer recruitment, as is the case for several species of numerically important gobies, or a prolonged recruitment period, as found for Atherina spp., the commercially important sea breams and sea bass largely recruit in the winter and spring at total lengths less than 2-3 cm. The latter species grow rapidly during the spring, summer and autumn, reaching total lengths of > 10 cm by September/October. In most cases there is a clear modal progression of the young of-the-year cohort that starts off as a uni-modal distribution. Over time, differences in individual growth rates and survivorship result in an increase in dispersion, with the transformation of the original uni-modal distribution to bi-modal or even multi-modal distributions by the autumn. As the water temperature drops, and with the intensive rainfall in the autumn, these species leave the Ria for the coastal zone, resulting in the minimum diversity observed in the winter.

The combined sampling with different gears, together with the results of the tagging study showed that fish distribution and abundance is highly dependent on habitat type. Thus, highest diversity and abundance was recorded for relatively shallow, protected areas with an abundance of sea grass. In contrast, the more dynamic zones of the Ria, such as the areas near the openings to the sea and those with sand or gravel bottom had the least diversity and abundance. Species composition was also a function of habitat type. The commercially important species of sea breams for example were most abundant in the sea grass beds, while certain species, especially flatfishes were largely found only in the outer sampling stations near the openings to the sea. In addition to differences due to habitat, other factors were also found to influence fish distribution and abundance. Thus at the species level there were important differences between day and night catches as well as between low and high tide. Sea grasses, namely Zostera spp., are clearly of fundamental importance for juveniles of a wide variety of species. Given their susceptibility to human impacts such as dredging and pollution, there is a need for monitoring and measures that will ensure the maintenance of healthy sea grass beds.

The tagging studies showed that survival of tagged sea breams of the species Diplodus vulgaris was high (99%) over a three month period in tanks. To date, we have had 305 (7.1%) recaptures since the start of tagging in September 2001. Unfortunately, there have been few tag returns from fishermen, even though considerable effort was spent in publicising out study and rewards were offered. Most of the recaptures were from our own field studies. A significant proportion of the tagged fish that were re-captured were caught at the same location where they had been tagged or nearby. Most recaptures took place on the same side of the channel where the fish had been tagged and released. This indicates high site fidelity, in particular to areas with sea grass beds. As the sea breams and bass grew, they tended to move in the direction of the openings to the sea. Since sea breams recruit to the artisanal fisheries in the coastal zone at two to four years of age, we still expect to get some returns in the future.

The comparison with the earlier study of Monteiro (1989) showed that overall, fish biomass as determined by standardised beach seining was not significantly different in 2001-2002. Some differences were found between the two periods in terms of the relative abundance of the most important species. For example, the European sea bass was apparently more abundant in 2001-2002 than in 1981-1986. For some species of sea breams the opposite was found. It general, it can be concluded that there has been no significant change in terms of diversity or total fish biomass in the Ria, which continues to be of vital importance as a nursery for many of the commercially important coastal species.

A long-term monitoring programme of the juveniles and of particular habitats should be initiated in the Ria Formosa. Our study can be used to determine the optimal sampling strategy. Such a monitoring programme will provide invaluable information on the dynamics of many commercially important species that have seen decreases in landings over the past 10 to 15 years. As an example, the development of a young-of-the-year index can be used to predict future catches and for management and conservation purposes. Furthermore, such monitoring can be used to evaluate possible changes due to global warming and/or the impacts caused by the increase in the human population and in development in the area surrounding and influencing the Ria Formosa.

Key words: Portugal, Algarve, Ria Formosa, Sparidae, settlement, recruitment, habitat use, tagging, GIS, critical habitat, essential fish habitat.

The report is available in PDF under request