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Impacts of small-scale fisheries on mangrove fish assemblages
Jose´ Amorim Reis-Filho
1,2,
*, Euan S. Harvey
3
, and Tommaso Giarrizzo
1
1
Laborato´rio de Biologia Pesqueira e Recurso Aqua´ticos, Universidade Federal do Para´, Av. Perimetral 2561, Terra Firme, Bele´m, PA 66040-170,
Brazil
2
Instituto de Biologia, Universidade Federal da Bahia, Campus Ondina, Salvador, BA, 40170-000, Brazil
3
School of Molecular and Life Sciences, Curtin University, Perth, WA 6845, Australia
*Corresponding author: tel: 55 71 999757465; e-mail: amorim_agua@yahoo.com.br.
Reis-Filho, J. A., Harvey, E. S., and Giarrizzo, T. Impacts of small-scale fisheries on mangrove fish assemblages. – ICES Journal of Marine Science,
doi:10.1093/icesjms/fsy110.
Received 13 December 2017; revised 1 August 2018; accepted 7 August 2018.
The data requirements and resources needed to develop effective indicators of fishing impacts on target stocks may often be great, especially
for mangrove fisheries where, for example, tidal cycles sequentially flood and drain the habitat as a result of natural processes. Here, we used
underwater video systems to evaluate the impact of small-scale fisheries on mangrove fish assemblages at four levels of fishing pressure (low,
medium, high, and no pressure). The lowest values of species richness and abundance were recorded in the areas fished most intensively.
Conversely, the highest species richness and the occurrence of larger-bodied fish were recorded in areas of reduced fishing activity, which was
surprisingly similar to the “no fishing” areas. The slopes of the community size spectra steepened in response to exploitation, while the relative
abundance of medium-sized fish (16–25 cm) declined. Fishing for local or regional markets, rather than subsistence, also led to a decrease in
the abundance of larger fish (>41 cm). The marked response of population parameters to fishing pressure reflected the impact of unregu-
lated small-scale fisheries on areas of mangroves. Fishery management practices that ignore contemporary changes in these environments are
likely to overestimate long-term yields, leading to overfishing. Thus, size-based approaches to evaluating fishing pressure were suitable for
detecting negative responses from the mangrove fish assemblages. A next step will be to integrate size- and species-based ecological
approaches that provide mechanisms to address pronounced decreases in specific species as a more profitable indicator of fishing impacts on
mangrove fish assemblages. This approach will allow the development of effective conservation and management strategies.
Keywords: fish community, fishing pressure, mangrove habitats, size spectra, subsistence fishing
Introduction
The importance of mangroves to the productivity of nearshore
fisheries has long been of considerable interest to marine ecologists
and policy makers (Lee, 2004;Manson et al.,2005). Mangroves are
thought to provide essential ecological functions by acting as a fil-
tering system and intercepting nutrients, pollutants, and suspended
matter from freshwater and/or tidal run-off (Marshall, 1994;
Valiela et al.,2001;Shervette et al.,2007;Reis-Filho and Santos,
2014). Mangrove systems also export organic matter and support
nearshore food webs (Sasekumar et al.,1992;Twilley et al.,1997;
Moura et al., 2011). In addition, mangroves are widely believed to
be critical to the productivity of coastal fisheries by acting as nurs-
ery areas for many target fish species (Manson et al.,2005). It is
also widely assumed that mangrove habitats contribute directly to
secondary productivity by harbouring the initial developmental
phases of commonly targeted species that migrate from mangroves
to off-shore, e.g. snappers, grunts, and sea-bass, thereby reinforcing
the productivity of commercial fisheries in a given area (Baran,
1999;Hutchison et al., 2014). It would thus seem reasonable to ex-
pect that the loss of mangrove habitat would lead to a reduction in
fishery productivity. Despite extensive research into the ecological
and management implications of these interrelationships between
mangrove habitats and their associated fish fauna (Barbier, 2000;
Blaber, 2000;Beck et al., 2001;Mumby et al.,2004;Sheaves et al.,
2014), the impacts of fishing on mangrove fish communities are
still poorly understood.
A literature review by Manson et al. (2005) showed that while
a large number of studies has focused on the relationships
V
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between mangroves and coastal fisheries (e.g. the abundance of
juveniles, and growth and survival rates), fishery data may not be
sufficiently accurate in spatial or quantitative terms to detect the
impacts of fishing on mangrove systems. Despite extensive re-
search on mangrove habitats and fishery interactions (de Graaf
and Xuan, 1998;Baran, 1999;Baran and Hambrey, 1999;
Loneragan et al., 2005;Islam and Ikejima, 2010), and several
reviews on this theme (Blaber, 2000;Sheridan and Hays, 2003;
Manson et al., 2005;Sheaves, 2017), few data are available on the
effects of fishing on mangrove fish assemblages. Most of the fish-
eries associated with mangrove systems are located in underdevel-
oped societies, where they are typically subject to little
management or planning (Barbier, 2000). In Brazil, in particular,
there are extensive areas of remote coastline where small-scale
fisheries operate with little effective regulation (see Reis-Filho and
Leduc, 2017), using a broad range of fishing equipment and tech-
niques to target the enormously diverse coastal–estuarine fish
fauna (Krumme et al., 2014), which hampers the effective man-
agement and monitoring of these small-scale catches and the reli-
able evaluation of the impact of fishing pressure in mangrove
areas.
Approximately 75% of the world’s fish harvest is obtained from
continental shelves or adjacent coastal and estuarine areas where
primary production is high (Pauly and Christensen, 1995;Pauly,
2008;Pauly et al.,2008). Coastal fishers use a variety of
different types of gear, many of which disturb habitats and cause
direct or indirect impacts on many fish species (Krumme et al.,
2014;Reis-Filho et al., 2016b). Thus, the stocks and catch per unit
effort of many target species have declined considerably in recent
years (FAO, 1997;Pauly and Zeller, 2016). There is worldwide
public concern that fisheries are depleting marine resources and
that fishery management is largely ineffective at halting this process
(Hutchings et al., 2010;FAO, 2011). Furthermore, studies
investigating subsistence fishing have concluded that at high levels,
it can affect the structure of marine fish assemblages (Jennings and
Polunin, 1996;Dulvy et al.,2004;Graham et al.,2005). The press-
ing need for understanding the ecological consequences of the re-
moval of harvested fish and the impacts of fishing activities on
community and ecosystem function has raised a large number of
questions on the sustainability of the current levels of fish harvest
(Messieh et al., 1991;Jones, 1992;Dayton et al.,1995;Roberts,
1995;Langton et al., 1996;Jennings and Kaiser, 1998).
Approximately 210 million people live in low-lying coastal
areas within 10 km of a mangrove forest, and many of these
inhabitants benefit from mangrove fisheries (Hutchison et al.,
2014). Despite these potential benefits, the direct effects of the re-
moval of fish (or fishing pressure) by mangrove fisheries are
poorly investigated. Identifying the critical threshold of fishing
intensity that an assemblage can withstand before ecosystem
function is affected is crucial to the development of effective indi-
cators for conservation and management planning and decision-
making (e.g. maintenance of biomass coupled with integrity of
community structure and function). Given this, the aims of this
study were to (i) evaluate the potential impacts of small-scale
fisheries operating in mangrove forests on the fish assemblage
structure and (ii) assess the utility of size spectra (SS) as a quanti-
tative indicator of the impact of fishing for monitoring and man-
agement purposes in tropical areas. The study is based on the
comparison of the ecological indices of the fish assemblages
found in mangrove areas that have different levels of fishing pres-
sure exerted upon them. We provide evidence to show that high-
medium pressure of small-scale fishing affects the fish assemblage
structure of mangrove sites and that a reduction in fishing inten-
sity should be considered as an effective fishery practice to attain
sustainable goals.
Material and methods
Study area and data collection
Video cameras were used to sample fish populations at 80 man-
grove sites located within Todos os Santos Bay, (TSB; 12500S,
38500W) in Bahia state, northeastern Brazil (Figure 1). This em-
bayment is the second largest coastal bay in Brazil and is domi-
nated by bi-directional currents, which are stronger during the
ebb tide in most of the bay (Lessa et al., 2001). The tidal regime is
characterized by symmetrical diurnal tides ranging between
2.1 and 2.4 m (Cirano and Lessa, 2007). The mean annual rainfall
of the study region is 2 400 mm, with a rainy season occurring
from March to August (mean monthly precipitation 280 mm)
and dry season from September to February (110 mm per
month). We conducted surveys in mangrove ecotones, located at
the edge of a number of contiguous mangrove forests dominated
by the prop roots of two species (Avicennia schaueriana and
Rhizophora mangle) and to a lesser extent, by Laguncularia race-
mosa (Costa et al., 2015).
All of the mangrove forests sampled in this study had low to
medium level of structural complexity, with similar parameters
[i.e. low mean diameter (D
130
: 3.15–5.87 cm) and height (3.65 6
1.35 m) and medium trunk density (23, 100–32 400 live
trunks ha
1
)]. A reduced degree of structural complexity was con-
firmed from local studies that have been performed in TSB (Costa
et al., 2015) and in comparison with the structural parameters of
other mangroves on the east coast of South America (Gomes
Soares et al.,2012). The area of mangroves at the different survey
sites varied from 130 to 350 m
2
. At all sites, the mangrove habitats
were bordered by well-defined, unvegetated channels that were 3–6
m wide, contained sand and clay, and became exposed at low tide.
Data collection—fish assemblages
The abundance of fish was estimated by the fish per minute (FM)
parameter, following the method of Reis-Filho et al. (2016a), dur-
ing the three peak days of each spring tide (n¼10) between
November 2013 and March 2014. The study focused on these
months because water visibility (up to 3 m) is best during the
dry season period. Records were not included in the analysis
when visibility was less than 1 m. The samples were recorded be-
tween 06:00 and 16:00 to avoid any potential effects of crepuscu-
lar or nocturnal behaviour (Reis-Filho et al., 2016a). The daily
timing of the samples guaranteed a similar distribution of sam-
pling effort throughout the entire study period. At each sampling
site, we set four wide-angle video cameras arranged randomly
along the edge of the mangrove at a height of approximately
20 cm above the substrate and positioned so that the front
and profile of the mangrove could be seen during filming
(Supplementary Figure S1a). The cameras were set at a 105angle
to permit the monitoring of a field of view from the water surface
down to the substrate (Supplementary Figure S1b), during the
entire tidal cycle with the depth in each sampling site varying
from 0.4 to 2.1 m. All video recordings were made with GoPro
cameras (GoPro
TM
Hero 3þHigh Definition camera). As the
cameras had a battery life of approximately 10 h (4 017 mAh lith-
ium batteries), they were redeployed daily. When the cameras
2J. A. Reis-Filho et al.
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were deployed initially, an acclimation period was allowed for by
discarding the first 10 min of the footage [as in the approach pro-
posed by Becker et al. (2012) and Reis-Filho et al. (2016a)]. Fish
counts were obtained only from the final 2 days of the video foot-
age to minimize any potential bias associated with the initial
deployment of the devices.
Data collection—fisheries activities
For the definition and measurement of the fishery pressure at each
sample site, we used data from a long-term fishery monitoring pro-
gramme that collected daily records in the TSB between 2010 and
2014. This monitoring programme assessed the multispecies catches
of the local commercial artisanal and subsistence fleet (Reis-Filho
and Oliveira, 2014;Reis-Filho et al.,2014,2016b;Reis-Filho and
Specht, 2015). The fishing activities monitored in the mangrove
forests and their edge included manual trawl nets of varying sizes
(10–30 m, with mesh sizes ranging from 12 to 20 mm), monofila-
ment gill nets in open water (with mesh sizes ranging from 10 to
40 mm), artisanal long lines (30–60 hooks), and stationary artisanal
fishing devices (e.g. “tapesteiros”). The majority of fishing activities
(approximately 80%) were restricted to the daytime, given that the
darkness impairs navigation in the shallow tidal channels used to
reach the fishing sites (Supplementary Tables S1–S3).
The principal type of fishing gear used in the areas of man-
grove (circa 90%) was the “tapesteiro,” which is composed of
monofilament gill nets and is 100–200 m in length with stretched
mesh sizes ranging from 20 to 30 mm. These nets were set parallel
to the edge of the mangrove, fixed to wooden stakes driven into
the sediment, which prevents any fish from escaping underneath
the net until the turn of the tide (Supplementary Figure S5 and
video showing fishing practices; https://vimeo.com/245834816).
In the middle of the ebb tide, the net was suspended near the for-
est canopy to catch the fish coming from the inner mangrove.
The gillnet remained suspended until the end of ebb tide, when
the fishers gather the fish caught in the net. Based on fishing ef-
fort of the “tapesteiros,” we defined four categories of fishing
intensity—high fishing intensity (HFI) when the area was fished
200–300 times (usually days) per year, medium fishing intensity
(MFI) when the area was fished 100–199 times per year, low fish-
ing intensity (LFI) when the area was fished less than 100 times
per year (see fishery effort data—Supplementary Table S3), in ad-
dition to a “no fishing” (NF) category for sites where no evidence
was found of fishing with tapesteiros. Local fishers were also inter-
viewed on the commercial value of each species caught in the
tapesteiros, with the catches being classified into three human-use
categories: (i) solely subsistence (consumed by the fishers),
(ii) local market (fish sold within the local communities), and
(iii) regional market (fish marketed outside the communities).
Analysis of video footage
The video footage was downloaded and viewed on a computer
using Adobe Premier v.6.0. Each hourly interval of the tidal cycle
was divided into three periods of 20 min, from which two ran-
dom subsamples of 5 min were obtained, grouped and quantified
as fish per minute (see Reis-Filho et al., 2016a). For this, we as-
sumed that there is a relationship between fish abundance (i.e.
Figure 1. Location of the 80 mangrove sites at which the cameras were deployed. The circles are coded according to the intensity of small-
scale fishing operations (see key) and dark patches are the mangroves forests.
Subsistence fishing’s impact on mangrove fish communities 3
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the number of individuals) and the duration of the sample, with
longer sampling periods rendering higher counts. The relative
density of each species was calculated using the MaxN index
(Priede et al., 1994) as measure of abundance, represented by FM.
This value was based on the screening of the 5 min subsamples,
during which the frame that contained the highest number of
individuals of each species was selected. These counts were con-
sidered to be representative of each subsample where different
frames within each 5 min subsamples were used for each species
recorded. As this method diminishes the possibility of counting
the same fish twice, it provides a conservative estimate of fish
abundance.
For the analysis of fish sizes, we estimated the body size (in 5-cm
intervals) of all individuals captured in the video footage following
the method of Bell et al. (1985) for underwater fish length assess-
ment. A 5-cm scale pipe was attached to an arm, 1.5 m in front of
the camera housing to provide a scale. For these measurements, an
entire fish had to be visible in the frame, approximately parallel to
the plastic pipe, and no more than 1 m behind this scale bar. The
person analysing the video footage was the same one who had pre-
viously set up the cameras in the water, so a 62cmerrorwastaken
into account in the evaluation of the video footage.
Data analysis
While all our surveys were conducted in similar mangrove habitats
dominated by A. schaueriana and R. mangle trees, our preliminary
results revealed considerable variability among sites in terms of the
abundance of fish. To minimize the effects of this variability (e.g.
local habitat features and passing shoals), the data for each sampled
site were expressed as a fraction of the maximum FM value ob-
served, i.e. FracFM ¼FM/maxFM, where FM is the total number
of fish observed per minute, and maxFM is the maximum FM
value recorded during the 5 min subsamples to all recorded species
(Ellis and Bell, 2008). The FracFM values thus range from 0 to 1,
with a value of 0 indicating no fish, and a value of 1 indicating the
highest number of individuals recorded in a 5 min subsample re-
gardless of the absolute abundances observed at a given site.
The mean FracFM values (for the whole fish assemblage and the
different groups of species of commercial value) from each man-
grove site were used to evaluate the differences in fishing pressure
categories. The variation in the mean total abundance and species
richness (number of species) among different fishing categories
(four levels: HFI, MFI, LFI, and NF) was analysed using a permuta-
tional multivariate analysis of variance, or PERMANOVA
(Anderson et al.,2001,2008). For this analysis, the significance
level (a) was set at 0.05, and statistical significance was tested using
9999 permutations of the residuals under a reduced model
(Freedman and Lane, 1983) and Type I (sequential) sum of squares
(Anderson et al., 2008) with Bray–Curtis dissimilarities. A permu-
tational analysis of multivariate dispersion (PERMDISP) was used
to test differences in the multivariate dispersion of groups of fish-
ing intensity categories. Similarly, a PERMANOVA was used to
evaluate whether the density of mangrove trees (ind ha
1
)varied
significantly among the different categories of fishing pressure.
We obtained size spectra (SS) to assess the effects of fishing on
mangrove fish assemblages. The SS, which are described by a de-
creasing linear function, were calculated by plotting the log
10
(xþ1) of the number of individuals in each 5-cm length class for
the whole fish assemblage and each specific group of fish for hu-
man use (i.e. subsistence, local, and regional market) against the
rescaled log
10
midpoint of each length class. After inspection of
the size range conforming to a linear regression (Figure 2), all fish
assemblages and distributions of the size classes along the gradi-
ent of fishing pressure were scaled. The resulting slopes and inter-
cepts (or midpoints) of the SS reflect the change in mortality
rates and the indirect effects of mortality (Graham et al., 2005),
and the intercept values reflect the spectrum of the size distribu-
tion and depend largely on changes in total abundance, while the
slopes reflect the results of reductions in abundance of specific
size classes experiencing fishing pressure. Moreover, the removal
of large fish by fisheries may have a direct effect, resulting in a re-
duction in the slope or an indirect effect by contributing to an in-
crease in the abundance of small prey species through predation
release, which would increase the slope (Shin et al., 2005).
To avoid the potential problem of correlation between the slopes
and the intercepts, the midpoints of the size classes were rescaled
according to the mid-length of each human-use category (fish
used solely for subsistence: range ¼5–25 cm, midpoint ¼18 cm;
fish marketed locally: range ¼41–60 cm, 50 cm; fish marketed re-
gionally: range ¼46–60, 52 cm), which was fixed at zero (Rochet
and Trenkel, 2003;Daan et al., 2005). The significance of the rela-
tionship between the slope or midpoint and fishing intensity was
then tested using a linear regression, and the variation among the
four fishing categories was evaluated using PERMANOVA, with
the different sites being treated as replicates. The significance of
the differences between the slope and the height of the midpoint
among fishing categories was tested using PERMDISP.
Finally, fine scale changes in each 5-cm body length class
(5–60 cm) were investigated by plotting the log
10
(xþ1) of the
numbers of individuals in each fish assemblage and the three
human-use categories against fishing intensity. The significance
of these relationships was also tested using linear regression.
Results
Effects of fishing on the regional fish assemblage
A total of 28 fish species were recorded by the video cameras.
Eleven of these species were found only at sites with LFI or NF
(Table 1). The total abundance of fish varied considerably among
the different fishing categories (Figure 2). Based on the FracFM
values, the highest mean fish abundance was recorded at the NF
Figure 2. Overall size spectra of fish assemblage separated by
commercial value category (black area ¼fish sold in the regional
market, checkered area ¼local market, open area ¼subsistence)
and entire fish assemblage (gray area).
4J. A. Reis-Filho et al.
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sites (FracFM ¼0.76), followed by the LFI categories (FracFM ¼
0.62). The mangroves with HFI and MFI presented similar
FracFM values (0.23 and 0.21, respectively). The effects of fishing
pressure were also observed in species richness, with the largest
number of species (28 in each case) being recorded in the NF and
LFI categories, with lower values being recorded in the MFI
(17 species) and HFI (13 species) categories. When the species
were separated by commercial value, the species marketed locally
and regionally were less abundant in the HFI and MFI categories
than in the LFI and NF categories (Figure 2). However, in the
case of the species solely used for subsistence, fishing intensity did
not appear to affect abundance. In general, abundance and diver-
sity did not appear to be affected at sites with LFI in comparison
with non-fishing areas.
Species richness (F¼6.78, p<0.03), overall fish abundance (F¼
9.45, p<0.02), and the abundance of the species of commercial
value (local market: F¼10.39, p<0.02; regional market: F¼10.08,
p<0.01) all varied significantly among fishing categories (Table 2).
These fish and fishery parameters thus appeared to be affected mini-
mallyincomparisonwiththeLFIandNFvalues(Figure 3).
However, the density of mangrove trees did not vary significantly
among the fishing intensity categories (univariate F¼2.16, p¼
0.92), which indicated that the mangrove forests are structurally simi-
lar in the different fishing grounds. The partitioning of the variance
indicated that the fishing pressure explains 52.9% and 61.3% of the
varianceinthemodelforthefishmarketedlocallyandregionally,re-
spectively. The PERMDISP found no significant difference between
the LFI and NF mangroves (F¼3.41, p¼0.08), reinforcing the sce-
nario that low levels of fishing may not affect the fish assemblages us-
ing mangrove areas, but the PERMDISP revealed significant
variation between the NF mangroves and the HFI (F¼12.31, p<
0.01) and MFI (F¼10.49, p<0.01) sites and between the LFI man-
groves and the HFI (F¼9.13, p¼0.02) and MFI (F¼8.19, p¼
0.02) sites (Figure 3).
Size-spectra profiles
The slopes of the SS varied significantly by fishing intensity cate-
gory (F
[6.83]
¼14.9, p<0.001) and human-use fish categories
(F
[5.41]
¼9.81, p<0.01) with the highest values being attributed
to the NF and LFI categories (Figure 4a). Fishing intensity was
correlated negatively with the slopes of SS (Figure 4c; F
[7.89]
¼
12.31, p<0.001). A similar, but less pronounced pattern, was ob-
served in the height of the SS, which varied significantly between
the fishing intensity (F
[5.98]
¼10.87, p<0.004) and human-use
categories (F
[4.29]
¼6.77, p<0.005) with a decline in abundance
towards the HFI category (F
[3.21]
¼5.49, p<0.04) (Figure 4b
and d). In general, the mean midpoint, slope, and height of the
SS declined significantly with fishing intensity.
Table 1. Mean abundance (fish per minute) and standard error (s.e.) of fish species recorded by the video cameras at the mangrove sites with
different levels of fishing intensity.
Species
Mean abundance, fish/min (s.e.) in the:
CategoryHFI sites MFI sites LFI sites NF sites
Eucinostomus melanopterus 9.7 (4.3) 9.2 (5.67) 13.3 (6.1) 12.6 (5.32) Subsistence
Eucinostomus gula 0.9 (0.48) 1.3 (0.65) 1.42 (0.98) 1.34 (0.72) Subsistence
Eucionostomus argenteus 2.31 (1.02) 2.69 (1.1) 3.99 (1.45) 4.09 (1.11) Subsistence
Diapterus rhombeus 1.34 (0.92) 2.99 (0.9) 7.71 (2.36) 7.98 (3.01) Local market
Gerres cinereus 0.98 (0.18) 1.91 (0.48) 4.76 (2.31) 6.69 (2.09) Local market
Eugerres brasilianus 0.13 (0.04) 0.87 (0.12) 3.87 (1.39) 4.18 (2.01) Regional market
Lutjanus jocu 1.76 (0.87) 1.98 (1.17) 3.14 (2.07) 6.19 (2.39) Regional market
Lutjanus alexandrei 1.59 (0.13) 2.09 (0.98) 4.98 (1.92) 5.82 (1.68) Regional market
Lutjanus cyanopterus – – 0.49 (0.09) 1.98 (0.61) Regional market
Atherinella brasiliensis 9.89 (7.45) 10.76 (5.21) 9.17 (5.99) 11.09 (5.19) Subsistence
Lile piquitinga 4.32 (1.72) 4.98 (2.31) 10.14 (5.98) 9.18 (6.72) Local market
Sphyraena barracuda 0.69 (0.05) 0.91 (0.72) 1.76 (0.6) 3.01 (1.78) Regional market
Centropomus undecimalis – – 1.22 (0.81) 2.76 (1.07) Regional market
Pomadasys corvaeniformes – – 1.03 (0.03) 1.19 (0.3) Subsistence
Pomadasys ramosus – – 1.87 (0.56) 2.13 (0.81) Subsistence
Archosargus rhomboidialis – 0.23 (0.03) 0.98 (0.21) 2.01 (0.34) Local market
Mugil sp. – – 2.01 (0.88) 4.05 (1.92) Regional market
Caranx latus – 0.61 (0.09) 1.39 (0.4) 2.18 (0.9) Regional market
Carangoides bartholomaei – – 0.88 (0.03) 1.73 (0.07) Regional market
Selene vomer – – 0.93 (0.03) 1.97 (0.39) Subsistence
Oligoplites saurus – 0.9 (0.08) 1.11 (0.2) 1.91 (0.7) Subsistence
Gymnotorax funebris – – 0.05 (0.01) 0.9 (0.04) Subsistence
Sphoeroides testudineus 4.67 (2.31) 4.22 (1.84) 5.08 (1.99) 5.18 (1.72) Subsistence
Bathygobius soporator 2.91 (1.45) 3.14 (2.08) 3.99 (1.33) 3.78 (1.04) –
Ablennes hians – 0.87 (0.31) 0.99 (0.5) 1.88 (0.3) –
Strongylura marina – – 0.75 (0.02) 0.9 (0.3) –
Haemulon parra – – 0.56 (0.32) 1.39 (0.1) Local market
Haemulon stendachneri – – 0.32 (0.04) 1.03 (0.02) Local market
Fish in the subsistence category were consumed in the fishers’ households, those in the local market category were sold in the local communities, and those in
the regional market category were sold outside the communities. The “–” indicates species not exploited by the fishers.
HFI, high fishing intensity; MFI, medium fishing intensity; LFI, low fishing intensity.
Subsistence fishing’s impact on mangrove fish communities 5
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The effects of both fishing intensity and human-use categories
varied among size classes (Figure 4). In the overall fish assemblage,
the abundance of the smaller size classes (<16 cm) tended to in-
crease slightly in response to fishing intensity (Figure 5a).
Conversely, the abundance of the larger size classes (>16 cm) de-
clined significantly (p<0.05, except for the 31–35 cm class) with
fishing intensity. In the case of the fish used for subsistence, we ob-
servedapositivetrendintheabundanceofthesmallestsizeclass
but a sharp decline in the 21–25 cm size class, although no signifi-
cant variation was detected (Figure 5b). For the fish marketed locally
(Figure 5c)andregionally(Figure 5d), a significantly negative rela-
tionship (p<0.05 in both cases) was found with fishing intensity.
Discussion
The results of this study indicate that small-scale fisheries have
substantial impacts on mangrove fish assemblages in terms of fish
Figure 3. Mean, standard error and total distribution of abundance of all fish recorded in the mangrove fishing zones (a), species sold in the
local market (b), species used for subsistence (home consumption) (c), and species sold in the regional market (d). HFI ¼High fishing
intensity, MFI ¼Medium fishing intensity, LFI ¼Low fishing intensity and NF ¼No fishing. Values in bracket are the % of proportion of total
abundance contributed by each of the categories.
Table 2. Results of the PERMANOVA of species richness and abundance (i.e. FracFM data based on fish per min) in response to four different
levels of fishing categories (high, medium, LFI, and NF).
Source df SS MS Pseudo-Fp
Square root
(components of variation) % of Variance
Species richness
Fishing intensity 3 14 599 14 599 6.78 0.023 18.21 39.13
Mean total abundance
Fishing intensity 3 23 763 23 763 9.45 0.017 13.23 47.54
Mean abundance of fish destined for subsistence
Fishing intensity 3 18 933 18 933 1.34 0.065 3.41 1.38
Mean abundance of fish destined for the local market
Fishing intensity 3 17 002 17 002 10.39 0.012 32.41 52.9
Mean abundance of fish destined for the regional market
Fishing intensity 3 18 344 18 344 10.08 0.009 28.76 61.3
Significant p-values (p<0.05) are shown in bold type. The variation attributable to each individual term is expressed as the square root of the components of
variation and the % of variance.
6J. A. Reis-Filho et al.
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abundance, species richness, and size metrics. Important differen-
ces were found between subsistence and commercial fisheries, as
well as in relation to the target market (e.g. local or regional).
However, low levels of fishing efforts have minimal impact on
species richness and abundance. In particular, the slope of the SS
and the abundance of the different fish size classes decreased with
increasing fishing pressure, and the midpoints were lower. If these
patterns reflect a causal relationship, then SS would be appropri-
ate indicators of the impact of fishing on mangrove fish assemb-
lages. In fact, the decline in the height of the SS and the
abundance of fish in response to fishing intensity may be a useful
indicator of the overall abundance–biomass effects of the multi-
species fishery activity in the mangrove. However, as the mid-
point heights were unrelated to the distribution of abundance
across the 5 cm size classes, it is important to analyse the slope
profiles by size class to determine how changes in abundance are
distributed across the size range analysed. In this case, the decline
in these parameters can be interpreted as the result of either
(i) an increase in the abundance of smaller-bodied individuals or
(ii) the depletion of larger individuals or species (Dulvy et al.,
2004). To differentiate between these alternatives, the effects of
fishing should be evaluated for each size class (Daan et al., 2005),
as a more pronounced decline in abundance would be expected
in the larger individuals. In addition, an increase was observed in
the relative number of individuals in the two smallest sizes classes
(5–10 and 11–15 cm) in the overall assemblage and the subsis-
tence group, indicating some form of indirect compensatory ef-
fect, such as reduction in predation pressure from larger fish
(Audzijonyte et al., 2013). These findings indicate that the height
and slope of the SS had two different effects of exploitation on
the fish community, although a reliable interpretation of these
effects is complicated (Dann et al., 2005).
Previous studies have shown that simple metrics used to assess
the effects of fishing intensity, such as size spectra, can provide
important insights into the impacts caused by fisheries, related to
fishing practices, catch patterns, and yields (Jennings and
Polunin, 1996;Graham et al., 2005;Henriques et al., 2014).
The body size parameters used to characterize changes in com-
munity structure have generally focused on shifts over time
(Duplisea et al., 1997;Bianchi et al., 2000), which are assumed to
be a reasonable measure of fishing intensity. Our findings offer an
additional perspective, similar to that of Dulvy et al. (2004), who
analysed the impact of fishing among spatially replicated units.
This study assumed implicitly that exploitation is the principal
factor structuring assemblages. While other factors may also be
important, including habitat clearance (Huxham et al., 2004), the
type of mangrove substrate (Reis-Filho and Santos, 2014), and
pollution (Shervette et al., 2007), our study sites were selected
carefully to avoid potential biases derived from these factors. The
interpretation of the findings of this study should nevertheless
take other potential factors, such as habitat complexity and re-
cruitment rates, into account.
The greater reduction in the larger size classes in comparison
with the smaller ones may be explained by the differential re-
sponse of larger fish to exploitation, related to ecological factors,
such as their trophic guilds. In this context, piscivorous fishes are
Figure 4. Relationships between the size spectra metrics and fishing intensity. (a) Slope and (b) height observed in all four fishing intensity
categories (mean 695% confidence interval). Solid bars ¼fish sold in the regional market, checkered bars ¼local market, open bars ¼
subsistence, hatched bars ¼undefined destination. (c) Slope and (d) height averaged over the 80 mangroves sites sampled in four fishing
intensity zones.
Subsistence fishing’s impact on mangrove fish communities 7
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important consumers of fish biomass, and predation is an impor-
tant source of mortality in fish communities, which is thought to
be a fundamental structuring force in mangrove habitats
(Hammerschlag et al., 2010). A size-spectrum approach, com-
bined with an analysis of other factors (e.g. migrations from adja-
cent habitats—see Reis-Filho et al., 2016a), may reflect
predation-based structuring mechanisms, given that larger fish
typically prey on smaller-bodied ones (Pope et al., 1994;Scharf
et al., 2000). In our case, the removal of the fish of the larger size
classes (e.g. Sphyraena barracuda,Centropomus undecimalis,
Strongylura marina, and Ablennes hians) from the studied areas in
Todos os Santos Bay likely resulted in reduced predation of the
smaller size classes. The impoverishment of the trophic structure
of a community resulting from a decline in predator abundance
Figure 5. Relationships between the abundance of fish in each 5-cm length class and fishing intensity in mangrove fishing grounds. (a) whole
fish assemblage considering all fish recorded; (b) fishes used solely for subsistence; (c) fishes marketed locally, and (d) fish marketed regionally.
The solid line indicates a significant regression at p <0.05. Note that the scale of the y axis differs among plots. Some length classes are
missing in (c) and (d) due to the lack of records. HFI ¼High fishing intensity, MFI ¼Medium fishing intensity, LFI ¼Low fishing intensity
and NF ¼No fishing.
8J. A. Reis-Filho et al.
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often result in weaker top-down control, leading to predator-
release processes at lower trophic levels, and increased susceptibil-
ity to anthropogenic interference (Britten et al., 2014). The results
of this study indicate the occurrence of “trophic downgrading” in
the mangroves with HFI, given the potential role of the larger-
bodied fish as predators. Some of the consequences of this pro-
cess, such as predator release (Rayner et al., 2007;Prugh et al.,
2009;Ferretti et al., 2010) and trophic cascades (Frank et al.,
2005;Baum and Worm, 2009), are well known in oceanic envi-
ronments. However, the consequences of these processes on tem-
poral dynamics and stability of communities are less well
understood, especially in the context of intertidal habitats, such
as mangrove forests. We, thus, provide the first empirical evi-
dence of shifts in the structure of mangrove fish assemblages ex-
posed to different levels of exploitation from small-scale fisheries,
demonstrating a progressive decline in the abundance of almost
all size classes with increasing fishing pressure.
In addition to the tapesteiros (i.e. more frequent fishing gear),
a number of different fishing techniques is practised within our
mangrove study sites (see Supplementary Tables S1–S3).
Considering these techniques is important, given that mangrove
fisheries are multispecies and are typically opportunistic, being
based on an array of fishing methods that target fish of a wide
range of sizes (Hutchison et al., 2014). Targeting a wide-range of
fish sizes may have synergistic effects, exacerbating the impact of
the removal of individuals and differences in size structure result-
ing from the fishing harvest. In this context, the destination of
the catches and the rationale of the fishery are important factors.
While artisanal fishers target a wide variety of fish species and
sizes (Bizzarro et al., 2009;Blythe et al., 2013;Reis-Filho, 2016;
Dacks et al., 2018), they tend to focus on larger-bodied specimens
of higher commercial value to guarantee their income. The SS
profiles of the mangrove areas targeted most intensively by com-
mercial fisheries (i.e. species sold at local and regional markets)
indicate more intense effects from combined fishing practices.
Thus, the SS help elucidate both the primary (tapesteiros) and sec-
ondary (manual trawling and gillnetting) and/or combined
impacts of fishing pressure. Furthermore, our study revealed that
low levels of fishing have a reduced effect on fish abundance or
size distributions, although constant and low-intensity subsis-
tence fishing may have an impact marine-associated food webs
(Martin et al., 2017), and we suggest that its effects will depend
on the type and intensity of fishing practices.
Other factors may also influence fishing pressure, such as the
availability of specific types of infrastructure and access to mar-
kets. Recent research has found an increase in fishing pressure
(Brewer et al., 2012) and reduced fish biomass (Cinner and
McClanahan, 2006;Cinner et al., 2013;Maire et al., 2016;
McClanahan et al., 2016) in areas closer to markets. While we did
not assess the features (e.g. distance and population size) most
commonly used as indicators of market access (Maire et al.,
2016), we did evaluate the selectivity of fishers in relation to the
size of the fish harvested for commercial purposes. As the princi-
pal type of fishing gear (tapesteiro) used in the study area target
small and large fish indiscriminately, the destination of the fish
(i.e. local or regional markets) may provide important insights
for the understanding of the relationship between catch effort
and its impacts. As fish are not always caught for subsistence, a
measure of the income obtained from fishing may provide a use-
ful indicator of the influence of market integration on the impact
of fishing activity within a given area.
As the impacts of fishing are tangible, they require real man-
agement actions. Effective governance, such as including changes
in the delimitation of fishing zones (Nunan, 2014), providing the
opportunity for fishers to organize their activities based on a re-
duction in fishing levels, or increasing mesh size, as shown here,
could be a very useful management strategy for mangrove fisher-
ies, which are typically small-scale, and make multiple contribu-
tions to the local economy (Jentoft, 2014;Weeratunge et al.,
2014). In this context, it is fundamentally important that fisheries
are able to manage stocks in a changing environment, and the
possibility of increasing mesh size would be one type of precau-
tionary as management action. Where practical, restrictions on
mesh size may be useful for the implementation of regulations on
minimum fish size, and management plans should also attempt
to include some age-related or developmental reference point. In
fact, as reliable biological information is available for most spe-
cies, optimal limits of fish size can be estimated that will maxi-
mize fish population biomass (Froese et al., 2008). In addition,
we recommend the immediate establishment of fishing zones
with stricter monitoring and enforcement. Any such initiative
may nevertheless be hampered if biological parameters cannot be
reconciled with the needs of fishing communities, such as the ex-
ploitation of commercial species (Hoyt, 2014;Mangubhai et al.,
2015). In the study of Jaiteh et al. (2016), fishers adapted to the
loss of fishing grounds through zoning by shifting operations to
alternative areas or diversifying their economic activities. In the
studied mangroves of TSB, the spatial and temporal zoning of
fishing grounds with the cooperation of the local fisher
communities could be a highly effective management strategy.
The importance of effective integration and the success of man-
agement practices increases with the number of fishers and scale
of fisheries in not only mangroves but also estuarine systems as
whole, which are fundamentally important on the Brazilian coast,
in terms of catch volume (Freire et al., 2015).
Fishery management based on the integration of fishers
through traditional management practices, co-management,
and/or government initiatives (McClanaham et al., 2006) must
include the establishment of reference points that the fisheries
can aim to meet. While long-term regional patterns are unknown,
Reis-Filho et al. (2016b) modelled the historical fishery scenarios
of the TSB and found that the intensity of exploitation using gill-
nets (which are similar to those used by the mangrove fisheries
studied here) has increased over time. Thus, our results also sug-
gest that the impacts of fishing on mangroves can depend of the
style and intensity of fishing practices over time (with reduced
effects at sites with lower fishing intensity) and the different uses
of the organisms targeted, for example, as food or for sale on local
and regional markets. Similarly, in the case of larger fish marked
for sale, the distance to the nearest market may often be a strong
driver of the characteristics of local fish assemblages (e.g. Cinner
and McClanahan, 2006;Brewer et al., 2012;Cinner et al., 2013),
and the fact that all the mangrove fishing grounds evaluated
in the TSB are located in close proximity to local markets
(see Caroso et al., 2011) may reinforce the pervasive impacts of
widespread subsistence small-scale fisheries. The sum of the evi-
dence indicates that the impacts of fishing pressure on fish stocks
make these populations more prone to abrupt shifts. However,
the fact that NF and LFI areas presented similar characteristics
also supports the discussion of management approaches that may
be more acceptable to local communities, given that the complete
closure of mangroves to fishing would not be necessary to
Subsistence fishing’s impact on mangrove fish communities 9
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guarantee the conservation of existing species assemblages. Thus,
the use of the “tapesteiro” and other types of gear, at moderate
levels, could be considered to achieve sustainable goals. To ensure
that mangrove fisheries are sustainable, management strategies
should focus on the relationship between the size structure of
stocks and their abundance, together with the integrated manage-
ment of fishing grounds, such as using closed seasons, or the rota-
tional exploitation of fishing zones. At a different social level, the
establishment of fishing rights at a local or community level has
proven to be a powerful tool for the protection of mangrove fish-
eries in many countries and has often resulted in the astute and
sustainable use of mangrove resources. Furthermore, the creation
of protected areas, combined invariably with the participation of
local communities in the development and implementation of
management measures, such as the rational use of fishery practi-
ces, would be an important step towards the sustainability of
mangrove fisheries.
Supplementary data
Supplementary material is available at the ICESJMS online ver-
sion of the manuscript.
Acknowledgements
We are indebted to the fishers of the Todos os Santos Bay who
allowed us to enter their household environments, and supported
our team during this study. We would also like to thank
Francisco Barros (UFBA) for their support during the writing
phase of this study. We thank the editor and the anonymous ref-
erees for their valuable comments on the manuscript. We are
grateful to ICHTUS Environmental Solutions for assistance and
advice in the field. This study was approved by the Federal
University of Bahia Animal Ethics Committee (20/2014), which
adheres to the Code of Practice of the Brazilian Federal
Government. This study complied with all government regula-
tions, including authorization for scientific activities from the
ICMBio (license 44139-1, authentic code 27479644).
Funding
J.A. Reis-Filho was funded by a PhD grant from CAPES
(Coordenac¸~ao de Aperfeic¸oamento de Pessoal de Nı´vel Superior).
T. Giarrizzo receives a productivity grant from CNPq (grant #
310299/2016-0) and a PNPD grant from CAPES.
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Handling editor: Jonathan Grabowski
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