Taxonomic distribution
Total of 212 fish species within 64 families were recorded from the ten TCEs covered in this study (Appendix E). Of these, 126 species were reported from at least one of the ten estuaries, and 44 species were found exclusively in a single estuary (PON). Five species, Etroplus suratensis, Pseudetroplus maculatus, Oreochromis mossambicus, Mugil cephalus, and Megalops cyprinoides, were common in all the TCEs. Cichlidae, Ambassidae, Megalopidae, Mugilidae, and Carangidae were the common families recorded in all the estuaries, with the number of families ranging from 17 (in KAD) to 50 (in PON).
Taxonomic similarity
Similarity in taxonomic composition between estuaries was determined using binary (presence/absence) and quantitative (percentage composition) data. The NMDS plot revealed the presence of four distinct clusters (Fig. 2). Similarly, the analysis of species’ presence/absence data based on cluster analysis showed a clear division into four separate groups of estuaries with a high level of dissimilarity between the groups (Fig. 3). First group comprised of two estuaries, PON and CHT (35% similarity in taxonomic composition), second cluster comprised of two estuaries (KOD and VEM), which were more similar in their species composition (55%), the third cluster comprised of three estuaries (AKA, POV and VEL (with 50% similarity) The fourth cluster included three estuaries, ANC, PT and KAD, where KAD and PT found more similar in species composition (60%), while ANC showed 45% similarity in taxonomic contribution.
Analysis of functional guilds
Estuarine use guilds
Contributions of estuarine use guilds varied between the estuaries (Fig. 4.A). Major guilds were MM (45.1%), ER (15.5 %), FS (18.8%), MS (14.8%) and FM (3.9%). The AN guild was absent in six estuaries (ANC, AKA, KOD, VEL and KAD) and shared least proportions (1.9%) to the fish community. Though the presence of FM guild was recorded in all estuaries, it contributed only to 3.8% of the fish assemblages. NMDS plot of estuarine use guild demonstrated a division into five clusters (Fig. 5), with ANC, AKA and POV forming the first cluster; KAD, VEL and PT the second cluster; and VEM and KOD forming the third cluster. Fourth and fifth clusters comprised of single estuary each, viz., CHT and PON.
Cluster analysis of estuarine use guilds demonstrated a clear division of estuaries into four groups with low dissimilarity (Fig. 6). First group comprised of three estuaries; KOD, CHT and VEM, all of which were relatively dissimilar within the group, the second cluster also included three estuaries, AKA, ANC and PT, which were similar in their ecological guild composition, the third cluster comprised of three estuaries; KAD, VEL and POV. A single estuary, PON formed the final cluster, which was dissimilar in guild composition compared to the other three clusters.
Feeding mode guilds
Relative proportions of feeding mode guilds varied among the estuaries (Fig. 4.B) - major guilds being PV (31.5%), ZB (24.6%), OV (16.8%), DV (11.4%), HV (9.5%) and ZP (6.3%). NMDS plot for feeding mode guilds revealed the presence of four clusters (Fig. 7). Estuaries POV, VEL, PT, ANC, AKA and KAD formed the first cluster and KOD and VEM comprised the second cluster. The third and fourth clusters consisted of single estuary each - CHT and PON, respectively. Cluster analysis of feeding mode guilds showed a clear division into four separate groups with high dissimilarity (Fig. 8). The first group consisted of four estuaries, CHT, KOD, VEM and AKA of which two viz., VEM and AKA were relatively similar within the group. Second cluster comprised of three estuaries, VEL, ANC and PT, which were identical in their feeding mode functional group distributions. Third cluster comprised of only two estuaries KAD and POV, while PON was the only estuary that was classified into the fourth cluster.
Analysis of habitat quality
EFCI was measured for defining the ecological quality of the ten TCEs considered in this study. Through this index, we established significant relationships between anthropogenic stressors and EFCI. In addition, scoring thresholds were also identified for each metric per estuary, representing significant variations with increasing stressors over time. Major anthropogenic factors and extent of each stressor, which influences the habitat quality of the TCEs of Kerala were also analyzed. In VEM, the major anthropogenic stressor was habitat loss (60–80%), in PON, destructive fishing and overexploitation contributed to 30–60% of the anthropogenic stressors, and in VEL, both habitat loss and water pollution were the major stressors (up to 60%). Barriers for fish migration was detected in CHT, KOD and VEM. Stressors such as tourism and recreation activities, and alien invasive species were observed in all TCEs. The pressure, from alien species invasion was highest in VEL. The composite pressure index (CPI) yielded a minimum score between 16 (KAD, ANC, POV, PT and AKA) and 24 (VEM) (Table 2). Estuaries along the northern coast had greater values of CPI than those along the southern coast. Simple regression analysis revealed that EFCI has a significant relationship with the pressure index (R2 = 0.31) (Fig. 9). Based on EFCI, it was realized that three estuaries, PT, AKA and POV were in “moderate” state; Four estuaries, VEM, PON, CHT and KOD were under ‘good” condition, while remaining three estuaries, ANC, VEL and KAD were in a “poor” state (Table 3).
Table 2
Description and values used to score anthropogenic pressures in the ten TCEs covered in the present study (codes as per Table 1).
Score
|
1
|
3
|
5
|
PON
|
CHT
|
KOD
|
VEM
|
VEL
|
KAD
|
AKA
|
ANC
|
PT
|
POV
|
Habitat loss (including reduction in bathymetry and topography (%)
|
10–30%
|
30–60%
|
< 60%
|
1
|
1
|
1
|
5
|
3
|
1
|
1
|
1
|
1
|
1
|
Water pollution, including nutrient enrichment and eutrophication (%)
|
10–30%
|
30–60%
|
< 60%
|
1
|
1
|
1
|
1
|
3
|
1
|
1
|
1
|
1
|
1
|
Destructive fishing and over exploitation (%)
|
10–30%
|
30–60%
|
< 60%
|
3
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
Climate change-induced pressures including extreme climatic events (%)
|
10–30%
|
30–60%
|
< 60%
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
Invasive Alien Species (%)
|
10–30%
|
30–60%
|
< 60%
|
1
|
1
|
1
|
1
|
3
|
1
|
1
|
1
|
1
|
1
|
Tourism and recreation
|
Absent
|
|
present
|
5
|
5
|
5
|
5
|
5
|
5
|
5
|
5
|
5
|
5
|
Barriers for fish migration
|
Absent
|
< 60% and ≥ 40%
|
present
|
1
|
5
|
5
|
5
|
1
|
1
|
1
|
1
|
1
|
1
|
Sand mining
|
Absent
|
< 13 and ≥ 8
|
present
|
5
|
5
|
5
|
5
|
5
|
5
|
5
|
5
|
5
|
5
|
Total score
|
|
|
|
18
|
20
|
20
|
24
|
22
|
16
|
16
|
16
|
16
|
16
|
Table 3
Description and values used to score estuarine fish community index (EFCI) and composite pressure index (CPI) from the ten TCEs
Name of Estuary
|
Rating
|
EFCI
|
CPI
|
Ponnani
|
Good
|
42
|
18
|
Chettuva
|
Good
|
54
|
20
|
Kodungallur-Azhikode
|
Poor
|
34
|
20
|
Vembanad
|
Good
|
44
|
24
|
Veli-Akkulam
|
Poor
|
52
|
22
|
Kadinamkulam
|
Poor
|
48
|
16
|
Akathumuri
|
Moderate
|
54
|
16
|
Anchuthengu
|
Poor
|
40
|
16
|
Poonthura
|
Moderate
|
38
|
16
|
Poovar
|
Moderate
|
36
|
16
|