4 Discussion
Climate change has both direct and indirect impacts on fish stocks which are
exploited commercially preferably for the livelihood. Direct effects act on physiology and behaviour of fishes and alter their growth, reproduction, mortality and
distribution. Indirect effects encompass events like alteration of aquatic productivity, biotic community structure and composition of the marine and estuarine
ecosystems on which fishes depend for food and survival. Changes in primary and
secondary production will obviously have a major effect on fisheries production,
but it is not possible in the current state of knowledge to make accurate quantitative
predictions of changes in global marine primary production solely due to climate
change-induced salinity alteration (Mitra 2013).
In this paper, we used condition factor as proxy to assess the impact of salinity on
the fish community of the lower Gangetic delta complex. Condition factor is an
indication of the well being of an organism and is based on the hypothesis that heavier
fish of a given length are in a better condition (Bagenal and Tesch 1978; Abowei and
George 2009). It has been used as an index of growth and feeding intensity (Fagade
1979; Abowei et al. 2009), decreases with increase in length (Bakare 1970; Fagade
1979; Abowei 2009) and also influences the reproductive cycle in fish.
Our first order analysis from the data sets of two fish landing stations (Diamond
Harbour in the western sector and Canning in the central sector of lower Gangetic
delta region) clearly reflect a pronounced variation between the two sectors.
Significantly higher values of condition index in fin fish were observed from the
fish catch of Diamond Harbour, where the aquatic phase is congenial in terms of
salinity (Mitra 2013). This congenial salinity may be attributed to Farakka barrage
discharge situated in the upstream region of Ganga–Bhagirathi–Hooghly river
system. 10-year surveys (1999–2008) on water discharge from Farakka dam
revealed an average discharge of (3.7 ± 1.15) × 10
3 m
3 s
−1 . Higher discharge
values were observed during the monsoon with an average of (3.81 ± 1.23) × 10
3
m
3 s
−1 , and the maximum of the order 4524 m
3 s
−1 during freshet (September).
Considerably lower discharge values were recorded during pre-monsoon with an
average of (1.18 ± 0.08) × 10
3 m
3 s
−1 , and the minimum of the order 846 m
3 s
−1
during May. During post-monsoon discharge, values were moderate with an
average of (1.98 ± 0.97) × 10
3 m
3 s
−1 as recorded by earlier workers (Mitra
2013). This hyposaline condition supports migration of T. ilisha for breeding in the
upstream region of Gangetic delta and also acts as the congenial nursery ground of
several species of commercially important fin fish that has been reflected through
relatively high condition factor values (Table 1).
In the central sector, the ingression of seawater and resultant salinity increase has
completely reversed the picture with relatively low condition index values of
economically important fin fish species (Table 1). The unavailability of fresh water
in this sector of lower Gangetic delta due siltation and blockage of Bidyadhari River
since the late fifteenth century (Mitra 2013; Mitra and Zaman 2014, 2015) may be
the primary cause of (i) reproductive failure of economically important fin fish
Monitoring Condition Factor of the Dominant Fin Fishes …
353
Climate change has both direct and indirect impacts on fish stocks which are
exploited commercially preferably for the livelihood. Direct effects act on physiology and behaviour of fishes and alter their growth, reproduction, mortality and
distribution. Indirect effects encompass events like alteration of aquatic productivity, biotic community structure and composition of the marine and estuarine
ecosystems on which fishes depend for food and survival. Changes in primary and
secondary production will obviously have a major effect on fisheries production,
but it is not possible in the current state of knowledge to make accurate quantitative
predictions of changes in global marine primary production solely due to climate
change-induced salinity alteration (Mitra 2013).
In this paper, we used condition factor as proxy to assess the impact of salinity on
the fish community of the lower Gangetic delta complex. Condition factor is an
indication of the well being of an organism and is based on the hypothesis that heavier
fish of a given length are in a better condition (Bagenal and Tesch 1978; Abowei and
George 2009). It has been used as an index of growth and feeding intensity (Fagade
1979; Abowei et al. 2009), decreases with increase in length (Bakare 1970; Fagade
1979; Abowei 2009) and also influences the reproductive cycle in fish.
Our first order analysis from the data sets of two fish landing stations (Diamond
Harbour in the western sector and Canning in the central sector of lower Gangetic
delta region) clearly reflect a pronounced variation between the two sectors.
Significantly higher values of condition index in fin fish were observed from the
fish catch of Diamond Harbour, where the aquatic phase is congenial in terms of
salinity (Mitra 2013). This congenial salinity may be attributed to Farakka barrage
discharge situated in the upstream region of Ganga–Bhagirathi–Hooghly river
system. 10-year surveys (1999–2008) on water discharge from Farakka dam
revealed an average discharge of (3.7 ± 1.15) × 10
3 m
3 s
−1 . Higher discharge
values were observed during the monsoon with an average of (3.81 ± 1.23) × 10
3
m
3 s
−1 , and the maximum of the order 4524 m
3 s
−1 during freshet (September).
Considerably lower discharge values were recorded during pre-monsoon with an
average of (1.18 ± 0.08) × 10
3 m
3 s
−1 , and the minimum of the order 846 m
3 s
−1
during May. During post-monsoon discharge, values were moderate with an
average of (1.98 ± 0.97) × 10
3 m
3 s
−1 as recorded by earlier workers (Mitra
2013). This hyposaline condition supports migration of T. ilisha for breeding in the
upstream region of Gangetic delta and also acts as the congenial nursery ground of
several species of commercially important fin fish that has been reflected through
relatively high condition factor values (Table 1).
In the central sector, the ingression of seawater and resultant salinity increase has
completely reversed the picture with relatively low condition index values of
economically important fin fish species (Table 1). The unavailability of fresh water
in this sector of lower Gangetic delta due siltation and blockage of Bidyadhari River
since the late fifteenth century (Mitra 2013; Mitra and Zaman 2014, 2015) may be
the primary cause of (i) reproductive failure of economically important fin fish
Monitoring Condition Factor of the Dominant Fin Fishes …
353