The Mozambique’s fisheries are divided in three subsectors: industrial, artisanal, and subsistence. The artisanal and subsistence fisheries contribute to the large amount of landings, and more than 2/3 of landings in the economic exclusive zone [1
]. Small-scale and subsistence small-medium pelagic fisheries have a major socio-economic role for coastal communities, comprising an important direct protein food source [2
]. The small-scale and subsistence fishing takes place both from shore and from canoes and dhow-type planked boats, mostly propelled by sails [3
]), and they almost exclusively target the nearshore waters, up to 40 m depth or less [4
]. Doherty et al. [5
] reconstructed Mozambique catches and estimated them to be between 55,000 and 64,000 t/year in 1950s, and between 120,000 and 130,000 t/year by the late 2000s. According to the National Institute of Fisheries Research (IIP) statistics, on average, the annual catch for the period of 2003–2006 of small-scale fishes was around 40,000 t/year [6
]). Given the great potential available in terms of small pelagic fish, it is believed that this estimated value is only what has been reported by the fisheries authorities, since most of them are not reported [1
]. In fact, Mozambique has a history of high under-reporting of catches, mainly due to the illegal, unreported, and undeclared (IUU) catches from the artisanal and subsistence fishing sector. Artisanal fishermen often fish during the closed season, and in protected areas when it is not allowed [8
]. In the country, there are around 120,000 fishers and 658 small-scale coastal landing sites. National fish stocks are under great pressure, and many fisheries show signs of overexploitation and the shallow coastal waters are severely overfished [1
Most marine species, including small pelagic fish, are distributed along to the Sofala bank, characterized by an extensive and wide shallow continental shelf with a high marine biodiversity [9
]. In Sofala bank, the Engraulidae catches are very abundant, and Thryssa vitrirostris
is considered a particularly important fishery resource [6
]. Based on spatial factors, seasonal catch rate, and demographic data from the beach-seine fishery, it is considered that the large fishery of the species in Mozambique occurs in the Sofala bank region. The core of the exploitable population is determined in Pebane, and the species seems to migrate towards the coast at the onset of the rainy season (November–February), and then northwards [6
]. T. vitrirostris
is a small pelagic fish that belongs to the Engraulidae family, inhabiting coastal and estuarine waters [10
] and forming large schools or shoals at depths ranging from 0 to 50 m (www.fishbase.org
, 9 January 2018). It distributes throughout the Indian Ocean, Madagascar and the coast of Africa from the Alfredo port to the north of the Persian Gulf, and along the coast of Pakistan and India ([11
, 9 January 2018).
According to [5
], T. vitrirostris
represents the largest source of income for most of the population that lives along the coast, as well as for the country’s economy, providing crucial social benefits to these communities. The few studies carried out in Mozambique related to this species have reported greater representation in artisanal catches, as well as in terms of bycatch in industrial and semi-industrial demersal shrimp fisheries, which represent the most frequent and most important fisheries in Mozambique, and so this aspect make this species an important source of animal protein for coastal communities, playing an important role for the diet and local economy [7
The few studies focused on T. vitrirostris
in Mozambique are: (i) the study of [14
], which is based on data that has been collected through scientific cruises to address aspects of the biology of the species in general; (ii) the study of [16
], which addresses aspects relating to temporal distribution, growth, and the reproductive biology of T. vitrirostris
on Zalala beach; (iii) and lastly, the study developed by [6
] that addresses the distribution and biology of T. vitrirostris
and other Engraulidae along the coast of Sofala bank, western Indian ocean. This latter study is one of the main references currently, in terms of information about this species, namely concerning the analysis of the influence of fishing mortality (F), on the dynamics of the explored stock along the Sofala bank. Nevertheless, due to continuously inconsistent time series records, such as demographic data, most studies that assess T. vitrirostris
are based on short-term and/or episodic data. T. vitrirostris
fishing activity is socio-economically important and it assures daily livelihood incomes and food. The monthly length frequency data obtained between 2009 and 2014 by the Institute of Fisheries Research were used to determine the biological growth parameters, mortality rates, and recruitment season, and to assess the fishery exploitation status of T. vitrirostris
The Mozambique coast is subject to intense fishing pressure by artisanal fishing [1
]. Generally, no scientific information is available to support fishery management or ecosystem ecological-based management, with consequent socio-economic and biodiversity impacts. This increases the impact of a rise in poverty, under the current scenario of an increase in population along estuarine and coastal areas [17
]. The analysis of length-frequency data is a reliable way to obtaining information regarding fishery status in tropical areas, and in underdeveloped poor countries where continuously monitoring programs often fail. Therefore, this study can contribute to support fishery management and sustainability of one of the most important small-pelagic subsistence/artisanal coastal fisheries, and consequently quality of life (socioeconomic) in the region. To the authors’ knowledge, the monthly time-series used in this study was one of the largest datasets available for Thryssa vitrirostris
(Engraulidae) for stock status assessment. As no discards were observed, we considered that the landings were representative of total catches.
To obtain a reliable estimate of growth and biological parameters of the population, the suitability of length-frequency data must be ascertained. The raw length-frequency data should exhibit peaks with apparent shifts in modal length over time [18
]. As revealed by yearly length-frequency monthly plots, the data met these criteria. The estimated growth parameters (L∞
= 25.1 cm; K = 0.41) differed from those found in a nearby beach at Zalala (L∞
= 22.26; K = 0.44; [19
]), and in grouped data of Sofala bank (L∞
=19; K = 0.66; [6
]), with the asymptotic length being markedly higher, while the growth rate was lower. According to [20
], the asymptotic length (L∞
) was basically influenced by food availability and population density, while the growth rate (K) was a parameter that was dependent on genetic and physiological factors, which can vary according to environmental fluctuations.
Environmental conditions, and namely, sea surface temperature (SST) can affect fish growth and recruitment, namely in small pelagic species [15
]). According to [24
], oceanographic conditions, such as SST, upwelling, current regimes, and wind conditions at Pebane and nearby areas (present study) were similar to environmental conditions recorded in other fisheries studies in the region [19
]. Thus, biological parameter differences (L∞
and K) estimated among studies should not be related to putative environmental causes.
Previous studies on the biology of T. vitrirostris
] were based on episodic length frequency data collected through discontinuous years and/or areas. In the present study, the observed maximum length-class sampled was 25 cm, which was close to the L∞
estimations. Thus, we believe that the growth parameter estimated in the present work, using continuous monthly surveys, can contribute to enhancing the knowledge of the species biology and support Mozambique fisheries managers. The L∞
value estimated corresponded to an increase in previous maximum L∞
off 2.5 cm in northeastern Mozambique [6
]. However, comparing to [13
], the L∞
herein estimated was 0.7 cm below the L∞
value estimated in the Maputo region (Southeaster Mozambique). According to [21
], the asymptotic growth (L∞
) and growth rate (K) parameters were inversely related, which means that the greater the asymptotic growth (L∞
), the lower the growth rate (K), and the higher the growth rate (K), the smaller the asymptotic growth. Overall the L∞
value was higher than in other studies conducted in northeastern Mozambique, while the growth rate value (K) was lower [7
]. The scientific cruise of [13
] was conducted with a pelagic trawl. The range of the length-size in [25
] did not include length classes below 8 cm, nor higher than 22 cm. The size of the trawl mesh was not mentioned in the report, although we can assume (as standard in a scientific survey) that a small mesh size was used in the cod end. Thus, it is a little speculative to debate whether the growth parameter differences among studies are due to fishing gear, since gear selectivity and fish size retained are linked. However, environmental conditions were conservative. Therefore, we cannot exclude the probability that gear or fishing technique differences could explain the regional differences in parameter estimations.
The estimated mortality rates in northeastern Mozambique (Z = 1.31, M = 0.92 and F = 0.39) were also different from those found by [26
] (Z = 2.30, M = 1, and M = 1.5), with the M value depending on the method used by the authors, respectively. Consequently, fishing mortality (F) estimated based on the difference between F = Z − M would be 1 and 0.8 for each M value estimated by [26
]. Therefore, it can be questioned why growth rate (K) and mortality rates in the present study are lower than recorded elsewhere in other close areas (for instance, mortality rate values were almost at half, compared to similar studies). The observed differences may be associated with the methods that were used to analyze the size-classes frequency data and the range size of the length across studies. For instance, [26
] estimated mortality using Monte Carlos simulations (to cope with scarce data), while in the present work, a six year monthly size-class time-series dataset was used. Another reason can be also linked to the data acquisition areas/surveys. [26
] sampled offshore areas and pooled demographic information (inshore and offshore) for the estimation of biological parameters and stock assessment models, including mortality rate. Nevertheless, the observed maximum length found by these authors and [19
] were lower than those found in this study. In fact, the demography differences found among studies are considerable, with modal length-class values of the present study being higher comparatively to former studies. Therefore, new estimates of mortality achieved are more accurate, because the monthly data have captured a more complete population size distribution.
VBF analysis clear showed several cohorts over the year. In fact, the monthly modal progression analysis represented by histograms reveals a clear upward trend in the growth of the observed cohorts. This is related to the growth rate profile in tropical areas, which is continuous throughout the year, whilst in temperate areas, the growth rate shows a seasonal trend [27
]. According to [26
], the main spawning activity of T. vitrirostris
extends from November to January, and July to August. However, latter study does not cover all of the months in the year. In the present study, the recruitment analysis based on species demography matches the results obtained by [26
] regarding the species recruitment season. The recruitment analysis showed two clear proxies recruitment peaks: the first peak of recruitment occurs from April to July, and the second recruitment peak from September to October. The ontogeny period of small pelagic larvae species is considered to be short [30
], with the warm temperatures contributing to reduce the larvae stage duration [31
]. Considering a lag among spawning periods, larvae growth, and the fishing recruitment of young of the year fish (length-classes of 4–5 cm), a time lag of 1.5 to 2.5 months between spawning and fish gear recruitment can be determined.
The natural mortality rate (M) was higher than the fishing mortality (F), when it was partly expected to be the opposite trend if the schooling social behavior of species that form groups, making them more vulnerable to fishing mortality, was accounted for. However, natural mortality is related to the biological characteristics of the species [32
]. Small pelagic fish species usually present a fast growth and a relatively short life cycle and higher natural mortality. The tropical fish tend to have high natural mortality rates (M) in relation to fishing mortality (F), with the natural mortality (M) not evidencing any relationship with the asymptotic size (L∞
) or the growth rate (K) [27
]. This can be related to the fact that, unlike fishing mortality, natural mortality is associated with predation and diseases, two factors that are not related to the age of individuals [33
The probability of capture analysis recorded a standard selectivity logistic curve for purse seine fisheries. In fact, more than 50% of undersized fish are retained above the 6 cm size-class. Considering the size of first maturity is 13 cm [26
], our results showed a large juvenile mortality for beach seine gear. The percentage of juvenile fish in catch (size-classes) comprises 54.2% of the catches. According to the current fishing legislation the allowable minimum mesh size to beach seining net used to capture this species is 30 mm. For this mesh size the estimated first capture length was Lc = 4.43 cm and the retention size of 50% of the capture is Lc50
= 5.39 cm. Thus, the mesh size used may be inadvisable to exploit the resource if we consider that the first maturation size is 13 cm, which is much larger than the size of the Lc50
, indicating that it is necessary to adapt the biology of the species (maturity size) to the selectivity of the fishing gear.
The estimated fishing exploitation rate value (Eest.
= 0.30) was below the Emax.
= 0.48) but above the optimal exploitation rate value (E50
= 0.28). This means that the exploitation pressure is above the value of E under which the stock has been reduced to 50% of its unexploited biomass. According to [32
], a fishing stock is considered to be at a sustainable exploitation level when the exploitation rate does not exceed 50% (E50
), the point at which natural mortality (M) and fishing mortality (F) are at equilibrium. Recent studies confirmed significant increases in the number of fishing gear and fishermen into Zambézia Province, Pebane, due to higher fishing yields expected to be obtained in this area [6
]. So, some concerns arise from the future of T. vitrirostris
artisanal fishing exploitation regimes in Sofala bank, namely in Pebane, an area where species forms a single demographic population and where the core of the exploitable population is found [6
Along Mozambique coastal fishing communities, T. vitrirostris
is captured with beach seine, and no discards occurs. Thus landings are representative of the catch. The fishing gear has low selectivity, as shown by the analysis of the capture probability. It is generally recognized that in areas where food security is poor, fisheries tend to be overexploited and too high a proportion of juveniles are caught, placing populations at risk. Yet T. vitrirostris
population seems healthy and sustainably produces recruits and catches. Such results are related with fishing exploitation regime. However, considering the Eest.
= 0.3 and Lc/L∞
= 0.24 values, the production rate falls in a exploitation regime that requires some concern on the part of the fishery managers [34
], namely, continuously monitoring of the fishery to ensure maintained fishing mortality rate at a steady state. Another concern is the amount of catches under small mesh sizes that can increase the risk of overexploitation [34
]. If fishing managers considered maintaining the fishing exploitation rate as it stands (Emax.
), an increase in mesh size can be enforced in advance as a precaution management approach aiming directly at reduce fishing mortality. In several parts of the world, small pelagic species under continuous stock assessment and enforced regulations have been reported to be at biological risk, due to high exploitation levels [26
]. Taken together, and considering the socio-economic importance of T. vitrirostris
, the currently exploitation regime might require the implementation of management measures to avoid biomass reduction to unsustainable levels. At the current exploitation stage, it is essential to continue with monitoring surveys and evaluate the risk associated with fishing effort increases as fishing precautionary approaches.