Scientific Article
ISSN 1678-2305 online version
BOLETIM DO INSTITUTO DE PESCA
Mesquita et al. Bol. Inst. Pesca 2019, 45(2): e423. DOI: 10.20950/1678-2305.2019.45.2.423 1/11
FISHERY PARAMETERS AND POPULATION DYNAMICS OF SILVER
CROAKER ON THE XINGU RIVER, BRAZILIAN AMAZON
ABSTRACT
The study analyzed the fishery parameters and population dynamics of the silver croaker,
Plagioscion squamosissimus, and its exploitation status, before the damming of the Xingu River
by the Belo Monte hydroelectric dam. Silver croaker was caught throughout the year, with a
total production of 239 tons. Estimated Catch per Unit Effort (CPUE) was 14.16 kg.fisher
-1
.day
-1
.
The mean price paid to the fishers for a kilogram of silver croaker ranged from US$1.89 to US$3.28.
Mean longevity estimated was 7.68 years. The total mortality (Z) was calculated at approximately
1.44 year
-1
, natural mortality (M) was 0.76 year
-1
, fishery mortality (F) was 0.68 year
-1
, and the current
exploitation rate (E) was 0.47 year
-1
. The study highlights the importance of P. squamosissimus as a
fishery resource in the Xingu region and provides important insights for the development of future
fishery management strategies and conservation of the species stocks.
Key words: hydropower; fish fauna; fishing resources; Amazonian; UHE Belo Monte.
PARÂMETROS PESQUEIROS E DINÂMICA POPULACIONAL DA PESCADA
BRANCA NO RIO XINGU, PARÁ, BRASIL
RESUMO
Este estudo analisa os parâmetros pesqueiros e dinâmica populacional da pescada branca,
Plagioscion squamosissimus, e seu status de explotação, antes do barramento do rio Xingu pela
hidrelétrica de Belo Monte. A pescada branca foi capturada durante todo o ano, com uma produção
total de 239 toneladas. A captura por Unidade de Esforço (CPUE) foi de 14,16 kg.pescador
-1
.dia
-1
.
O preço médio pago aos pescadores por um quilo de pescada branca variou entre R$ 4,16e R$ 7,21.
A longevidade média estimada para a espécie foi de 7,68 anos. A mortalidade total (Z) foi calculada
em aproximadamente 1,44 ano
-1
, a mortalidade natural (M) foi de 0,76 ano
-1
, a mortalidade da
pesca (F) foi de 0,68 ano
-1
e a taxa de exploração atual (E) foi de 0,47 ano
-1
. O estudo destaca
a importância de P. squamosissimus como recurso pesqueiro na região do Xingu, e fornece
informações importantes para o desenvolvimento de futuras estratégias de manejo pesqueiro e
conservação dos estoques de espécies.
Palavras-chave: energia hidroelétrica; peixes; recursos pesqueiros; Amazônia; UHE Belo Monte.
INTRODUCTION
Traditionally, Amazonian fisheries are multispecific, and caught an extensive range
of fish species (Bayley and Petrere, 1989), although the activity is relatively selective,
with most effort being focused on few fish species (Barthem and Fabré, 2004; Hallwass
and Silvano 2016). The rivers of the Amazon basin can be divided into three types,
based on the physical-chemical properties of their water – whitewater, blackwater, and
clearwater (Sioli, 1968). While considered to be oligotrophic, the clearwater rivers have
an enormous potential for the generation of hydroelectric energy, and are subject to
extensive environmental impacts, as observed in the Tucuruí and Estreito hydroelectric
plants, in the Tocantins basin (Fearnside, 2001; Hallwass et al., 2013), Belo Monte on
the Xingu River (Fearnside, 2006) and the installations planned for the Tapajós River
(Fearnside, 2015). While less productive than whitewater rivers, clearwater systems
support important local fisheries, which are fundamental to the survival of riverside
Esther Mirian Cardoso Mesquita
1,2
Rivetla Edipo Araújo Cruz
1,2
Gustavo Hallwass
3
Victoria Judith Isaac
1,2
1
Universidade Federal do Pará – UFPA, Instituto de Ciências
Biológicas, Programa de Pós-graduação em Ecologia
Aquática e Pesca – PPGEAP, Rua Augusto Corrêa, 1,
Campus Universitário do Guamá, CEP 66075-110,
Belém, PA, Brasil. E-mail: esther_shekinah@hotmail.com
(corresponding author).
2
Universidade Federal do Pará – UFPA, Laboratório de
Biologia Pesqueira e Manejo dos Recursos Aquáticos,
Grupo de Ecologia e Manejo da Pesca na Amazônia
– GEMPA, Cidade Universitária Prof. José da Silveira
Neto, Avenida Perimetral, 2651, CEP 66070-530,
Belém, PA, Brasil.
3
Universidade Federal do Oeste do Pará – UFOPA,
Rodovia PA-254, 257, Santíssimo, CEP 68270-000,
Oriximiná, PA, Brasil.
Received: June 10, 2018
Approved January 25, 2019
FISHERY PARAMETERS AND POPULATION…
Mesquita et al. Bol. Inst. Pesca 2019, 45(2): e423. DOI: 10.20950/1678-2305.2019.45.2.423 2/11
populations. The silver croaker is widely targeted on the region’s
principal clearwater rivers, the Tocantins (Cetra and Petrere Junior,
2001) and the Xingu (Isaac et al., 2015).
Records of fishery catches provide a primary source of data
for the study of fish ecology and the status of stocks as well as
information on the composition, size, and numbers of fish harvested,
and fluctuations related to alterations in the environment (Barthem
and Fabre, 2004). In broad terms, fishers exploit natural systems
like predatory piscivores, and the analysis of fishery dynamics
provides essential insights into the natural fluctuations of fishery
resources and the relative abundance of the different stocks.
The understanding of the dynamics of fishery parameters also
permits the discussion of the factors that account for the behavior
of fishers, and their selection of target species (Batista et al., 2012).
On the Xingu, the construction of the Belo Monte Hydroelectric
power station (UHE Belo Monte), is likely to have a significant
impact on the fish fauna of all the environments affected.
In particular, Plagioscion squamosissimus is one of the five
species most targeted by local fisheries (Isaac et al., 2015). It has
been predicted that the change from a lotic environment in the
region of the reservoir to a lentic one may have a positive impact
on the abundance of the species, at least during the initial stages
of the damming of the river. This positive impact is expected
because the species is not migratory and is known to be highly
flexible in ecological and trophic terms (Hahn et al., 1997, 1999;
Stefani and Rocha, 2009), contributing to its capacity to adapt to
impacted environments (Lowe-Mcconnell, 1999). On the other
hand, the popularity of the species in local markets, and the
overall increase in demand for fishery products resulting from the
increase in the local population, associated with the installation
of the hydroelectric plant, may lead to an increase in fishery
pressure, which may ultimately result in the overexploitation of
P. squamosissimus stocks (Eletrobras, 2008; Isaac et al., 2015).
Overall, then, it will be essential to understanding whether or not
these two processes will cancel one another out, over the long
term, thus guaranteeing the sustainability of the region’s fisheries.
The South American silver croaker, Plagioscion squamosissimus,
Heckel, 1840, (Perciformes, Sciaenidae) is one of the principal
targets of extractive freshwater fisheries in the region, providing 5%
of total catches (Brasil, 2011). P. squamosissimus is a benthopelagic
species initially limited to the Río Orinoco and Río Amazonas
basins and rivers of the Guianas (Casatti, 2005), although it has
been introduced into areas outside its natural range, being found
in many South American reservoirs. In the Amazon basin, the
species is found in lakes and along river margins. The adults may
exceed 6 kg, with a total length of 70 cm (Silva, 1981).
The juveniles of the smaller size classes feed on Diptera larvae
(Chaoboridae and Chironomidae) and other aquatic insects
(immature forms) (Stefani and Rocha, 2009), while the adults
are predominantly piscivorous, characterizing a wide spectrum of
food items in the young and a more specialized, piscivorous diet
in the adults. The species has external fecundation and piecemeal
spawning, reproducing throughout the year, but most intensively
during the flood period (Soares et al., 2008). P. squamosissimus
is considered to be a sedentary species, given that no systematic
trophic or reproductive migrations have been observed (Granado-
Lorencio et al., 2005).
Any evaluation of the magnitude of the impacts of a hydroelectric
project on the local fish fauna will depend fundamentally on
previous knowledge of the biology and population dynamics of
the species before the effects. Data from the preceding period are
essential, for example, to assess whether the size of a population
has been altered, and to what extent fisheries have affected the
density of stocks and the viability of fisheries following the
modifications of the aquatic environment (Agostinho et al., 1997).
Population parameters of P. squamosissimus have been recorded
at the Barra Bonita reservoir in the Brazilian state of São Paulo
(Braga, 1998; Castro, 1999), at other reservoirs in the state of
Paraná (Gubiani et al., 2009), and at the Tucuruí reservoir, in
the Tocantins-Araguaia basin (Brambilla et al., 2015). Studies
in natural environments have included the Orinoco and Apure
rivers in Venezuela (González, 2005a, 2005b; Perez-Lozano
and Aniello, 2013), and whitewater rivers in the Brazilian
Amazon basin (Worthmann, 1980, 1987; Ruffino and Isaac 1995;
Cella-Ribeiro et al., 2015; Lima et al., 2017). However, no data
are available on the stocks or population dynamics of the silver
croaker in the Xingu River, before the installation of the Belo
Monte hydroelectric plant.
Therefore, the fishing focus on a small group of target species,
combined with the major environmental impacts currently affecting
the Amazon region, such as the widespread deforestation, and the
construction of hydroelectric reservoirs, are likely to have profound
implications for the equilibrium of its fishery stocks. However, few
systematic data are available on the populations of the principal
target species, given the lack of continuous fishery monitoring
or studies on the biology of the fish species. Data on population
dynamics and fishery parameters are essential for the development
of effective management practices for the maintenance of fishery
stocks (Schaefer, 1954; Mace, 2001; Hilborn, 2007). Given this,
the present study evaluated the population dynamics and fishery
parameter of P. squamosissimus, as well as the exploitation status
of its stocks on the Xingu River in the period before the damming
of the Belo Monte reservoir. The findings will be used to discuss
possible future scenarios for the species related to the potential
impacts and the novel environmental conditions created by the
damming of the river.
MATERIAL AND METHODS
Study area
The Xingu River is more than 2.300 km long, and its principal
tributary is the Iriri River, followed by the Bacajá (Figure 1).
The Xingu discharges into the Amazon River in the Brazilian
state of Pará and has a basin of more than 500.000 km
2
. The part
of this basin located in Pará covers 24.5% of the total area of the
state (Sepaq, 2008). The fluvial regime of the middle and lower
Xingu is characterized by a flood period between December and
February, high water in March and April, the receding period
between May and June, and low water between August and
November (Eletrobras, 2011).
FISHERY PARAMETERS AND POPULATION…
Mesquita et al. Bol. Inst. Pesca 2019, 45(2): e423. DOI: 10.20950/1678-2305.2019.45.2.423 3/11
The Xingu River is distinct from most other tributaries of the
Amazon, due to its restricted floodplain and abundance of islands.
The topological diversity found along the Xingu River contributes
to its unique configuration, creating distinct environments,
such as anastomosed channels, rapids, cascades, and waterfalls.
In addition to the existence of fluvial plains, which are wider or
narrower, depending on the slope and velocity of the river, as
well as the features associated with the flood period, when the
riparian forests are inundated, forming “igapós” or blackwater
swamps (Eletrobras, 2008). Cyclical fluctuations in the level of
the river and the backwaters found along its margins are related
to the seasonal variation in precipitation and have a profound
impact on the behavior of many species, in particular, fish.
This variation also determines the spatiotemporal dynamics of
the local fisheries (Isaac et al., 2015).
Data collection
The data were collected at 21 fishing ports on the middle and
lower Xingu River, arranged in nine localities (Figure 1). All the
catches were recorded daily (Monday through Saturday) at these
ports between September 2012 to August 2014, by trained workers.
During this period, a total of 7688 fishing trips were monitored.
During the landing of the catches, the data collectors interviewed
the fishers or owners of all the boats that arrived to land their
catches. During these interviews, information was obtained on
the total production per species, the type of vessel and fishing
technique used, the period, fishing ground and the type of
environment, and the mean price of the catch.
Data on the total length (TL) of the P. squamosissimus specimens
were obtained through the random selection of individual during
the landing of the catches at the ports of Altamira, Vila Nova,
Vitória do Xingu, Belo Monte, and Senador José Porfirio, with a
total of 7776 specimens being measured. Data on the discharge
of the Xingu River, measured at the town of Altamira, during
the study period, were obtained from the records compiled by
Norte Energia, S.A.
Data analysis
Fisheries
The descriptive statistics of the catches included: total production,
fishery effort (number of fishers and number of days fishing),
the type of vessel, the mean price of the first sell of the product,
total income, and the mean CPUE (Catch Per Unit Effort), that is,
productivity, per month, year or season. To analyses the mean price
and total income we did the currency conversion from Brazilian
Real (R$) to American Dollar ($), using the dollar quotation to
the studied period, thereby $1,00 equivalent R$ 2,20.
The fishery productivity was evaluated based on estimates of
the CPUE 2, calculated using Petrere Junior et al. (2010) equation.
Figure 1. The river sectors and localities at which the fishery catches were monitored on the Xingu River (State of Pará, Brazil)
from September 2012 to August 2014.
FISHERY PARAMETERS AND POPULATION…
Mesquita et al. Bol. Inst. Pesca 2019, 45(2): e423. DOI: 10.20950/1678-2305.2019.45.2.423 4/11
2
Captura total (Kg)
CPUE =
(n° de pescadores x n° dias de pesca)
(1)
To calculate the CPUE, the data from the three principal kinds
of “fishery production systems”. The “fishery production system”
is the combination of the type of vessel and fishing equipment,
which was used here to standardize the fishing effort and the
effect of selectivity (Gulland, 1956). The systems of fishery
production considered were the canoes with long-tailed outboards
and nets, canoes with longtailed outboards and lines, and canoes
with long-tailed outboards and nets and lines, which together,
represent 74% of the total croaker catch.
For the analysis, the catches in which the silver croaker contributed
more than 40% of the total of catch, which assumes a degree of
selectivity (even though the fisheries are essentially multi-specific),
were considered (Cruz et al., 2017). The nonparametric Kruskal
Wallis test was used to evaluate the variation in productivity
according to the fishery system and season, with a 5% significance
level and multiple comparison tests.
Population Dynamics
For analysis, the measurements of the specimens were
grouped in monthly frequency classes, with a 3 cm total length
interval. The mean, minimum, and maximum lengths were also
calculated, with the respective standard deviations. The analyses
of population dynamics were run in FISAT II, using the Electronic
Length-Frequency Analysis (ELEFAN I) approach (Pauly and
David, 1981), which is based on the modal shift of the temporal
sequences of the length data to adjust them to von Bertalanffy’s
somatic growth model.
( )
0
-K t-t
Lt=L 1-e



(2)
Longevity was calculated using the formula of Taylor (1960).
The total mortality rate (Z) was calculated in FISAT II, based on
the linearized capture curve, converted to body lengths (Pauly,
1980). Pauly, (1980) empirical equation was used to calculate the
natural mortality rate (M), where the mean annual temperature
of the surface water was considered to be 28 °C.
( ) ( ) ( ) ( )
Log M = -0,0066-0,279 Log L +0,6543Log K +0,463Log
(3)
The natural (M) and total (Z) mortality rates were used to
calculate the fishery mortality (F = Z-M), and the exploitation rate
(E = F/Z), described by Baranov (1918 apud Sparre and Venema,
1997). The model of the yield per recruit of Beverton and Holt
(1957) was used here, assuming a “knife edge” type of selectivity,
with the parameters being estimated using a macro programmed
in Excel. The parameters L∞, K, M and F, obtained above, were
used to develop this model, with the value of W∞ being calculated
based on the weight x length ratio estimated for the species by
Giarrizzo et al. (2015). This approach determines the exploitation
rate, obtained for the maximum yield (E
max
), with E
50
being the
rate at which the spawning biomass would be 50% of the virgin
biomass, while E
0.1
represents the optimal exploitation rate when
the slope of the model in 10% of the initial slope.
RESULTS
Total catches and fishery effort
During the study period, the silver croaker catches represented
as much as 20% of the total fishery catch landed on the Xingu
River. A total catch of 239 (t) of croaker was recorded during the
7688 trips monitored during the two years of the study period
(2012-2014), with the largest monthly catch being obtained in March
2013 (19.8 t), followed by April and May of the same year. These
months correspond to the high-water period. The smallest catch
(3.2 t) was recorded in December 2012, followed by November
and December 2013, which correspond to the beginning of the
flood period (Figure 2).
The total fishing effort over the study period consisted of
19,892 days of fishing, involving and 10,737 fishers. Effort
peaked in March 2013, with 887 fishers, and 1413 days of fishing
(Figure 2), while the lower effort was recorded in December
2013, with only 167 fishers and 322 days. The amount of the
total catches landed was highly correlated with fishery effort
(Figure 2), regarding both the number de fishers (r
2
= 0.87) and the
days spent fishing (r
2
= 0.81). The mean (± SD) catch landed was
30±40 kg per fishing trip. The average trip lasted 2.64±1.9 days,
with crews of between one and nine fishers (Table 1).
Characteristics of the fishery fleet
The fishery fleet that targets silver croaker on the Xingu River is
made up of approximately 1005 vessels, which can be divided into
three types: (i) canoes with paddles, (ii) canoes with long-tailed
outboards, and (iii) motorboats. A total of 68 wooden canoes with
paddles were registered during the study. These were the smallest
vessels, with a mean length of 4.0 m (SD = 1.0 m), and the mean
capacity for the storage of 12 kg (SD = 14 kg) of ice. The bulk of
the fleet is made up of canoes with long-tailed outboard motors,
known locally as “rabetas,” a term that will be adopted here to
denominate the canoes with long-tailed outboards. The fleet
had 839 rabetas, with a mean length of 7.2 m (SD = 1.0 m) and
a mean ice capacity of 70 kg (SD = 75 kg). These vessels were
powered by motors of 5.5 to 7.5 Hp. The motorboats (a total of
98 units) were also made of wood, and were the fleet’s largest
vessels, with a mean length of 10.0 m (SD = 2.0 m), and a much
larger ice storage capacity (192 kg; SD = 260 kg). These boats
were powered by motors ranging from 10 Hp to 90 Hp.
The rabetas were the most important productive units in the
croaker fishery, being responsible for 85% of all the fishing trips
recorded, and 80% of the silver croaker catch. Together, the other
canoes and the motorboats contributed 15% of the total number
of trips, and 20% of the catch landed. The motorboats undertook
the most extended trips with the largest number of fishers, given
the autonomy of the vessels, and their storage capacity (Table 1).
Fishing techniques
Lines and gill nets were the main techniques used to target the
silver croaker, corresponding to 37% (line) and 32% (gillnet)
of the total catch, and 2766 and 2930 of the catches landed,