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Assessing Low Value Crustacean Bycatch Species Using Length Based Bayesian Biomass (LBB) Method, a Tool for Data Poor Fish Stock Assessment

Authors:
  • Central Marine Fisheries Research Institute, Veraval, Gujarat
  • ICAR-Central Marine Fisheries Research Institute

Abstract

The majority of tropical fish stocks lack sufficient data for conventional fish stock assessment, making them data-poor fisheries. The status of stock assessment is even more dismal for the low value fishes or crustaceans landed by the trawlers in a significant quantity. Crustaceans like non-edible small crabs (Charybdis spp) and stomatopods form a significant component of the low-value bycatch landed along the northwest coast. Despite the high ecological importance of these groups and the recent declining trend in catches (2007-19), no attempts so far have been made to evaluate the stock status of these groups from the study region. As reliable time series catch and effort data for the individual species are not available, a recently developed length-based approach, LBB (Length Based Bayesian Biomass) estimation method is adopted for the present study. Two of the evaluated stocks, Charybdis hoplites and Miyakella nepa were found abundant (B/BMSY>1.1), whereas Oratosquillina interrupta (B/BMSY=0.94) was found slightly overfished. The sufficient number of larger individuals were found lacking in all three species (L95th/L∞<<1.0). A higher incidence of juveniles in catches was estimated for C. hoplites and O. interrupta (Lmean/Lopt<<1.0).
Turk. J. Fish.& Aquat. Sci. 23(SI), TRJFAS22189
https://doi.org/10.4194/TRJFAS22189
Published by Central Fisheries Research Institute (SUMAE) Trabzon, Türkiye
P R O O F
R E S E A R C H P A P E R
Assessing Low Value Crustacean Bycatch Species Using Length
Based Bayesian Biomass (LBB) Method, a Tool for Data Poor
Fish Stock Assessment
Rajan Kumar1,*, Dineshbabu A.P.1, Shikha Rahangdale1, Vinay Kumar Vase1,
Jayshree Gohel1, Vipul Solanki1
1Indian Council of Agricultural Research-Central Marine Fisheries Research Institute, Kochi-682018, India
Article History
Received 05 July 2022
Accepted 29 November 2022
First Online 23 December 2022
Corresponding Author
Tel.: +918976694333
E-mail: rajan.kumar@icar.gov.in
Keywords
Arabian Sea
Sustainability
Trawl fishery
LBB
Bycatch
Abstract
The majority of tropical fish stocks lack sufficient data for conventional fish stock
assessment, making them data-poor fisheries. The status of stock assessment is even
more dismal for the low value fishes or crustaceans landed by the trawlers in a
significant quantity. Crustaceans like non-edible small crabs (Charybdis spp) and
stomatopods form a significant component of the low-value bycatch landed along the
northwest coast. Despite the high ecological importance of these groups and the
recent declining trend in catches (2007-19), no attempts so far have been made to
evaluate the stock status of these groups from the study region. As reliable time series
catch and effort data for the individual species are not available, a recently developed
length-based approach, LBB (Length Based Bayesian Biomass) estimation method is
adopted for the present study. Two of the evaluated stocks, Charybdis hoplites and
Miyakella nepa were found abundant (B/BMSY>1.1), whereas Oratosquillina interrupta
(B/BMSY=0.94) was found slightly overfished. The sufficient number of larger individuals
were found lacking in all three species (L95th/L<<1.0). A higher incidence of juveniles
in catches was estimated for C. hoplites and O. interrupta (Lmean/Lopt<<1.0).
assessed, which also includes non-commercial but
ecologically important resources having significant
bearing the overall sustainability of marine fisheries
(Baran, 2002; Link, 2007). Several methods have been
developed to address the issue of the assessment of
data-limited fishery. A comprehensive review of data-
limited methods along with proposal of new approaches
has been presented by Carruthers et al. (2014). A more
recent review (ICES, 2014; Rosenberg et al., 2014) has
found the approach proposed by Martell and Froese
(2013) promising to overcome the bottlenecks in the
assessment of data-poor fisheries, especially in tropical
waters. Further, Froese et al. (2017) improved upon the
method proposed by Martell and Froese (2013)
How to cite
Kumar, R., Dineshbabu A.P., Rahanglade, R., Vase, V.K., Gohel, J., Solanki, V. (2023). Assessing Low Value Crustacean Bycatch Species Using Length
Based Bayesian Biomass (LBB) Method, a Tool for Data Poor Fish Stock Assessment. Turkish Journal of Fisheries and Aquatic Sciences, 23(SI),
TRJFAS22189. https://doi.org/10.4194/TRJFAS22189
Turkish Journal of Fisheries & Aquatic Sciences
TRJFAS22189
Crustaceans, especially the non-edible crabs
(Charybdis spp) and stomatopods, are key ecological
species landed in significant quantity along the Indian
coast by the trawlers as a low-value bycatch (LVB) or
trash (Sukumaran, 1988; Dineshbabu et al., 2012; Pillai
et al., 2014; Dineshbabu et al., 2018, Kumar et al., 2019).
Smaller demersal crabs are known to be the major diet
component of several demersal fishery resources
(Philip, 1998; Xue et al., 2005; Abdurahiman et al.,
2010). The most important demersal crab in terms of its
ubiquitous distribution along the eastern Arabian Sea is
Charybis hoplites (Wood-Mason, 1877) (Dineshbabu et
al., 2018). Further, it is also the major crab species
featuring in the bycatch landings of the trawlers along
the Indian coast (Dineshbabu et al., 2012; Pillai et al.,
2014). Despite its high ecological importance and
significant fishing mortality (bycatch and discards), no
attempt to date has been made to assess its stock status.
Stomatopods are specialized marine crustaceans
broadly categorized in to two groups, smashers and
spearers (Ahyong, 2001). The latter, owing to their less
aggressive nature, are known to form large aggregation
(Caldwell and Dingle, 1975) and are among the most
abundant crustaceans in the tropical continental
ecosystem, regularly featuring the landings of bottom
trawlers (Antony et al., 2010). The trawlers operating
along the Indian coast are known to land large quantities
of stomatopods and a significant quantity also goes as
discards, especially the ones caught by multi-day
trawlers (Menon, 1996; Dineshbabu et al., 2012).
Several studies on the diversity and distribution of
stomatopods have been carried out (e.g., Manning,
1995; Ahyong, 2001, 2002, 2004; Ahyong and Kumar,
2018), but the studies on population dynamics (e.g.
Abello and Martin, 1993; James and Thirumilu 1993;
Kaiser et al., 2021) are limited and on stock status are
almost absent.
The north-west (NW) coast of India, along the
northeastern Arabian Sea, is known for the rich fishing
ground for predatory demersal resources. The sustained
productivity of the northern Arabian Sea throughout the
year has ensured the availability of demersal resources
in the region for commercial harvest (Parulekar et al.,
1982). The geo-morphology-induced high productivity in
the region coupled with easy access to rich fishing
grounds owing to wide continental shelves (shallow
depth over an extensive area) has permitted the rapid
development of trawl fisheries in the region since its
introduction in the 1960s. The development at a rapid
pace raised fears about the sustainability of the trawl
fisheries of the state and the region is currently having
the issue of overcapacity (Pravin et al., 1999).
Sathianandan et al. (2021) found that the North West
Coast (NW) of India is the most exploited coastal zone
along the Indian coast, with 54.2% of assessed stocks
falling into the 'overexploited' category. This can be
attributed to the largest trawl fleet (43%) along the
entire Indian coastline (CMFRI, FSI and DoF, 2020),
which also means higher landings of the bycatch
Turkish Journal of Fisheries & Aquatic Sciences
TRJFAS22189
the present assessment. The carapace width (CW) was
recorded for the crab (C. hoplites) and total length (TL)
for stomatopods (M. nepa and O. interrupta). All the
primary data were collected in a participatory mode
with fishers and lengths were recorded before the
sorting of the data to ensure that the LF reflects the
original population structure of the stocks.
Data Analysis Framework
Two of the recently developed and widely adopted
approaches for the assessment of data-limited fishers
are CMSY (Froese et al., 2017) and Length based
Bayesian biomass estimation (LBB) approach (Froese et
al., 2018). The former method requires time series catch
and reliance as input to arrive at stock status. However,
in tropical continental fishers as in the present case, the
catch data segregated up to individual species over a
longer temporal scale are rarely available. The presence
of similar-looking congeners in multi-species fisheries
makes the species-specific catch enumeration by field
surveyors a challenging task. Hence, the CMSY approach
probably could be applied in several fisheries of the
region, owing to lack of suitable data. LBB provides an
alternate analytical framework to access data-limited
(or data-poor) fisheries using length-frequency data
(either single year or time series) as input. It applies a
Bayesian Monte Carlo Markov Chain (MCMC) routine to
estimate indicators of stock status (Froese et al., 2018).
The method requires a set of priors for asymptotic
length (L), length at first capture (LC), and relative
natural mortality (M/K). The method gives users the
option to opt for the default prior or to provide a robust
Figure 1. The study area comprising of the coastal waters of Gujarat and Maharashtra, the two maritime state (~ provinces) of
India along NE Arabian Sea.
Turkish Journal of Fisheries & Aquatic Sciences
TRJFAS22189
F/M are available (Froese et al., 2018). The CPUE is
known to be proportional to the biomass of the
exploited portion of the population.
󰆒
󰆒

󰇡
󰇢
󰇡
󰇢

󰇡
󰇢

󰇡
󰇢

The relative biomass in the exploited phase of the
population, baring the fishing activities (F=0) can be
calculated as:
󰆒
󰇡
󰇢
󰇡
󰇢

󰇡
󰇢

󰇡
󰇢

Where B’0>Lc represents the exploitable fraction
(>Lc) of unfished biomass (B0). The index for the relative
biomass depletion for the exploited portion of the
population (B/B0) can be estimated as per Beverton and
Holt (1966) using the function:

󰆒
A proxy for relative biomass at maximum
sustainable yield (BMSY/B0) can be obtained by fixing
parameters F/M=1 and Lc=Lc_opt and recalculating
expression for Y’/R, CPUE/R’, B’0>Lc /R, and B/B0
(equation 5 to 7). Subsequently, the current relative
stock size (B/BMSY) can also be estimated as a ratio of
B/B0 and BMSY/B0, which indicates the stock status
(Healthy- B/BMSY>1.1; slightly overfished/ fully
exploited- 0.8<B/BMSY>1.1; overfished- 0.5<B/BMSY>0.8;
grossly overfished-0.2<B/BMSY>0.5; collapsed-
B/BMSY<0.2) according to Palomares et al. (2018) and
Amorim et al. (2019). The analysis presented here was
done using R code (LBB_33a. R), available at
http://oceanrep.geomar.de/44832/.
Table 1. Details of primary data and priors used for LBB estimation of the low-value crustacean landed by trawlers operated along
NE Arabian Sea.
Species
C. hoplites
M. nepa
O. interrupta
Length range (mm)
13-66 (DW)
41-122 (TL)
42-173 (TL)
Length class interval (mm)
4
5
10
L (mm)
7.13
14
19.5
M/K
1.5
1.5
1.5
Z/K
2
2.6
2.5
F/K
0.49
1.13
1.04
LC
2.45
6.88
7.14
Alpha
15.9
16.3
18.9
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(Table 1). Further, a sufficient number of larger
individuals in the population is also lacking, as indicated
by L95th/L (=0.89) of less than unity.
Miyakella nepa
M. nepa has been recorded in the size range of 41-
122 mm TL (Table 1) and has an estimated asymptotic
length of 14.2 cm TL (Table 2 and Figure 2B). The
currently estimated biomass was 52% of the virgin
biomass (B/B0=0.52). The stock was found abundant o as
B/BMSY (1.50) was higher than 1.10. Also, the relative
fishing mortality (F/M=0.53) was much lower than the
unity, further emphasizing the healthy status of the
stock. The Lmean/Lopt (0.98) and LC/Lopt (1.10) values were
very close to the unity. However, L95th/L (0.85) was
Figure 2. Graphical output LBB analysis of the three species: 2A - Charybdis hoplites; 2B Miyakella nepa; 2C Oratosquillina
interrupta.
Turkish Journal of Fisheries & Aquatic Sciences
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biomass in the tropical continental shelf ecosystem
plays a vital role in prey-predator interaction. Several
studies along the eastern Arabian Sea have highlighted
the importance of crustaceans in the diet of the
commercial fishery resources and hence link the
sustainability of the commercial fish stocks to the
healthy biomass of crustaceans’ prey, which includes
low-value crustaceans like small non-edible crabs and
stomatopods (Suseelan and Nair, 1969; Thangavelu et
al., 2012, Mohamed, 2004; Mohamed and Zacharia,
2009; Vase et al., 2021). Abdurahiman et al. (2010) in
thier feeding ecology study along the Arabian Sea, found
crabs and stomatopods as a key diet component of
demersal predator fish species.
The crab landings of Gujarat over the period of
2007-19 showed a gradual decline with the highest
recorded during 2008 (26283 t) and lowest during 2019
(7501 t) (Figure 3). The Catch composition of crab
landings of Gujarat during 2019 showed that Charybdis
spp (multi-species low-value bycatch component) forms
the major component, which includes non-edible crabs
or juveniles of commercially important crabs. Charybdis
hoplites were found to be the most dominant
component of the multi-species Charybdis spp complex
(Figure 4). The other species include C. callianasa, C.
omanensis, C. smithii, C. lucifera, C. helleri, etc. The
multi-day trawlers accounted for over 75% of the total
crab landings of Gujarat. Dineshbabu et al. (2018) also
found C. hoplites to be the most dominant component
Figure 3. Time series catch data of crabs landed along Gujarat coast during 2007-19 [Source: NMFDC of ICAR-CMFRI].
Table 2. Stock status of three major bycatch crustacean species landed along NE Arabian Sea based on LBB estimation approach.
Species
C. hoplites
M. nepa
O. interrupta
B/BMSY
1.2
1.5
0.94
B/B0
0.45
0.52
0.34
F/M
0.54
0.53
0.83
F/K
0.82
0.92
1.3
Z/K
2.4
2.6
2.9
Lmean/Lopt
0.75
0.98
0.8
LC/Lopt
0.62
1.1
0.68
L95th/L
0.89
0.85
0.86
L
7.22
14.2
19.7
Remarks
Healthy/Abundant
Healthy/Abundant
Fully exploited/ slightly overfished
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overfished) status indicated by a drastic drop in
landings. The two possible reasons which can be
attributed to the decline in landings of crabs over the
years, despite the healthy stock abundance, are the
increasing duration of fishing voyages and the diversion
of trawl effort towards pelagic trawling. The studies
conducted by Dineshbabu et al. (2012) found Charybdis
spp (non-edible) to have high discard rates in multi-day
trawlers operated along the eastern Arabian Sea.
Continuous Increase in duration of fishing by multi-day
trawlers over the last decade has led to more quantum
of discards than landings of these low-valued bycatch
components as retaining these low-value components
(non-edible crabs) over a longer period on-board is not
perceived economical by the fishers. Another major shift
observed in the trawl fishery of the study region is the
Figure 4. Catch composition of crab landings of Gujarat in 2019 [Source: NMFDC of ICAR-CMFRI].
Figure 5. Time series catch data of stomatopods landed along Gujarat coast during 2007-19 [Source: NMFDC of ICAR-CMFRI].
Table 3. Top five fishery resources landed by single day trawlers (MTN) operated along NW coast of India in terms of catch rates in
2019 [Source: NMFDC of ICAR-CMFRI]
Resource
Catch rates (kgs/hrs)
Non-penaeid prawns
105.17
Ribbonfishes
9.74
Croakers
7.79
Penaeid prawns
7.39
Stomatopods
3.47
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Stomatopods are another group of crustaceans
which are ubiquitously distributed in the tropical marine
ecosystem, have high ecological importance and forms
significant component of bottom trawl harvest (Caldwell
et al., 1989, Mohamed, 2004; Antony et al., 2010).
Several species of stomatopods in different parts of the
world forms seafood of high commercial importance
(Abello and Martin, 1993; Arshad et al., 2015; Kaiser et
al., 2021). However, along the Indian coast, they
historically form one of the major components of single-
day bottom trawl landings and discard from multi-day
trawlers (Kurup et al., 1987; Sukumaran, 1988; James
and Thirumilu, 1993, Menon et al., 1996; Dineshbabu et
al., 2012; Kumar et al., 2019). The stomatopods landings
of Gujarat have a declining trend during 2008-13.
However, a gradual increase in landings since 2013 is
evident in the catch trend (Figure 5). The gear
contributing to the stomatopod fishery of Gujarat are
multi-day trawlers (MDTN, 45.70%), mechanized
dolnetters (MDOL, 38.86%) and single day trawlers
(MTN, 15.44%). It is worth mentioning here that the
effort spent by MTN in hours is only 1.2% of MDTN and
5.4% of trawlers. Despite the low share in total effort
Figure 6. Gearwise landings of stomatopods along Gujarat coast during 2007-19 (MDOL- mechanized dolnets; MDTN multi-day
trawlers; MTN single day trawlers) [Source: NMFDC of ICAR-CMFRI].
Figure 7. Time series (2007-19) catch per unit effort realized for stomatopods in mechanized dolnets of Gujarat [Source: NMFDC
of ICAR-CMFRI].
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the trawls, which could potentially be the reason for the
stock to survive the wrath of the largest trawl fleet of
India (Gujarat alone accounts for 1/3rd of total trawl
operated along Indian coast) (CMFRI, FSI and DoF, 2020).
The presence of a sufficient number of larger
individuals in the population provides resilience to the
stocks against higher fishing pressure and bestows a
buffer against environmental fluctuations (Xu et al.,
2013; Le Bris et al., 2015). For all the three evaluated
stocks, there seems a lack of a sufficient number of
larger individuals (L95th/L<<1.0), which do not present a
healthy sign for long-term sustainability when climate
change in the Arabian Sea is real and a force to reckon
with (Piontkovski and Al-Oufi, 2015). Link (2007) opined
that several non-commercial species can be “Keystone
species” in an ecosystem and declining stock status of
these groups can lead to irreversible damage to the
entire ecosystem. Heavy exploitation of these groups
undermining their scientific management may have a
detrimental impact on the productivity of an otherwise
rich continental shelf ecosystem (Baran, 2002). Hence,
periodic monitoring and assessment of these
ecologically important organisms landed in significant
quantity are recommended to ensure the long-term
sustainability of the marine fishiness of the NE Arabian
Sea.
Conclusion
The present study is the first attempt in the Indian
context and among a few global attempts to evaluate
the stock status of non-edible portunid crab and
stomatopods. The tropical multi-gear and multi-species
fisheries are often considered data-poor. The data
became even scarcer when we think of low-value
bycatch (LVB) components. The catch data for the LVB
component are seldom segregated up to species level,
making species-specific assessment using conventional
methods almost impossible. The present study applied
an LBB approach which uses only LF data as input, which
can be collected with a fair degree of accuracy. The
present estimate found C. hoplites and M. nepa in a
state of abundance, whereas O. interrupta was found
fully exploited. Although M. nepa stock is abundant,
escalating the fishing pressure towards this resource is
not-recommended as similar species (O. interrupta)
having common fishing ground and mostly similar
catchability falls under the category of fully exploited.
Any increase in an effort towards the harvest of M. nepa
could potentially endanger the sustainability O.
interrupta and hence a “status quo” is recommended as
far as stomatopod fishery is concerned. Allowing a
sufficient number of larger specimens in the population
is also recommended, which can be achieved by
returning larger specimens (terminal length groups)
back to the sea. This measure would not be an economic
compromise as there does not exist any price
preference for a larger specimen. A better utilization
scheme must be formulated for these species, especially
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... To some extent, these methods have alleviated the pressure on fishery resource assessment due to the lack of data to some extent (Goodwin et al., 2006;Cope and Punt, 2009;Punt et al., 2011;Phillips et al., 2015;Armelloni et al., 2021). Among these, the electronic length-frequency analysis (ELEFAN) and length-based Bayesian biomass estimation method (LBB) are the simplest in terms of data requirements, requiring only representative length-frequency data (LFD) to estimate biological parameters and population resource status (Froese et al., 2018;Barman et al., 2021;Zhang et al., 2021a;Al-Mamun et al., 2022;Kumar et al., 2022). These methods offer a rational basis for the sustainable development of commercial fisheries and effective fishery management (Cui et al., 2020;Ju et al., 2020). ...
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