Habitat, growth and life history of the goby Parapocryptes serperaster

Author: Minh Quang Dinh

Dinh, Minh Quang, 2015 Habitat, growth and life history of the goby Parapocryptes serperaster, Flinders University, School of Biological Sciences

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Parapocryptes serperaster (Gobiidae) is a burrowing fish widely distributed in the Indo-West Pacific region, including the Mekong Delta. Although P. serperaster is commercially important, little is known of its habitat, growth pattern and reproduction. The thesis aims to provide fundamental information on (1) habitat use, (2) growth pattern and body condition, (3) food and feeding habit, (4) reproductive biology, and (5) population structure of this gobiid based on studies from June 2012 to March 2015 in the Mekong Delta, Vietnam. The results improve the understanding of the biology, fishery management and aquaculture of this gobiid fish.

The habitat of P. serperaster was investigated by examining the burrow morphology using resin castings in situ to obtain the physical structure and configuration of the burrow. The larger fish could make more sophisticated burrows and the burrow dimensions were positively correlated with fish size. Burrow structure and dimensions were similar between dry and wet seasons. In the laboratory, P. serperaster actively excavated burrows using body movement, which is an important adaptation for living in a shallow and muddy habitat. The burrow provides a retreat to protect gobies from predation, while its use for spawning and feeding were not found.

The morphometric and growth characteristics of P. serperaster were investigated in both dry and wet seasons. Both male and female gobies shared a similar growth pattern between the wet and dry seasons, and the slope of the length-weight regression was close to the threshold of isometric growth (i.e., 3). The body condition factor decreased as fish grew, and seasonal differences in the condition factor depended on the gender. The condition factor of male P. serperaster was greater than of females in the wet season, but similar in the dry season.

Diet composition and feeding habits of P. serperaster were investigated on different sizes of fish. The gut length was positively related to fish length and the relative gut length to total length ratio was 1.57±0.30, falling into the category of omnivorous fish. Its diet composition and diet breadth varied between seasons and fish sizes, and smaller fish had a wider diet breadth than larger individuals. This goby was a generalist feeder, mainly feeding on detritus, followed by diatoms. Its feeding intensity was higher in the wet season than in the dry season, but was not affected by fish size. P. serperaster fed Navicula spp. in the wet season, but Nitzschia spp. in the dry season.

P. serperaster was a multiple spawner as the ovaries contained oocytes at different developmental stages during the spawning season. The peak spawning period occurred in September when the gonadosomatic index was highest. Fish reproduction was characterised by high variation of fecundity and heterogeneous distribution of egg sizes in the ovary, indicating a possible adaptation of this goby species to the monsoonal climate.

The population biology and age structure of P. serperaster were studied based on monthly samples over a year. Fish age was determined using fish length distribution and otolith dimensions. The size of fish at first entry to the fishery catch was 14.6 cm and fish longevity was 4.05 yr. Fishing, natural and total mortalities were 1.57 yr-1, 1.51 yr-1 and 3.07 yr-1 respectively. The goby stock was not subject to overfishing since the exploitation rate (0.49) was lower than the maximum exploitation yield (0.83). Proper otolith morphometry of female and male gobies was used for age identification, and the age obtained from reading otolith annual rings matched the age determination from length frequency distribution.

This thesis provides new knowledge of habitat use, growth and life cycle of P. serperaster in tropical regions. The burrowing activity is an adaptation of fish living in shallow and muddy habitats, and the growth pattern provides useful information to understand gobiid morphometric variation in areas with monsoonal seasonality. The results of food and feeding habits, spawning pattern and reproductive biology contribute to knowledge of feeding adaptation of small-bodied bottom dwelling gobies to the mudflat habitat. The biovolumetric method is considered a reliable approach to quantify dietary composition in gobiid fish, and the use of otolith morphometry is a useful tool for age determination for gobies in tropical areas.

Keywords: Parapocryptes serperaster, burrow structure, isometric growth, condition factor, dietary composition, omnivore, reproduction, fecundity, exploitation, otolith

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2015
School: School of Biological Sciences
Supervisor: Prof. Jian Guang Qin