Date of Award

2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Biological and Environmental Sciences

Specialization

Sustainable Agriculture and Food Systems

Department

Fisheries, Animal and Veterinary Science

First Advisor

Austin T. Humphries

Abstract

Many local fishermen in Ghana depend on the sardinella fishery for their livelihoods. Flat sardinella (Sardinella maderensis) and round sardinella (Sardinella aurita) are the two main fish species that are exploited in the fishery. Landings of the sardinella stocks, however, have been declining for over a decade now as a result of overfishing engendered by overcapacity, open access fishery and weak governance. The focus of this work was to chart the path towards a sustainable management of the fishery in Ghana by identifying the key extractive components of the fishery, ascertaining the interactions among them and the fish stocks and using the life history of the fish to simulate their population dynamics under different scenarios of seasonal closures in conjunction with gear restrictions so as to arrive at the optimal management strategy.

Manuscript I: The goal of this study was to describe the basic fisher and catch characteristics as a first step in understanding the fishery and setting the basis for management decisions. We sampled 8 sites along the coast of Ghana from 2017-2018 and collected data on over 14,000 individuals of sardinellas from 332 unique fishing trips. We found three broad categories of fishing gears: beach seines, purse seines and gill nets. Three types of purse seines were identified and distinguished based on mesh sizes: poli ≤ 3.0 cm; watsa > 3.0 cm; and poli-watsa had a combination of mesh sizes. Differentiation of gill nets was based on either monofilament (called “set net”) or cotton (called “ali”). Poli-watsa constituted the most dominant gear in the fishery. Two species of sardinella were captured, the round (Sardinella aurita) and the flat (Sardinella maderensis) sardinella. Ninety-six percent of the vessels were motorized. The landings were predominantly juveniles. Considering the dominance of juveniles in the landings of the sardinella fishery in Ghana, management that reduces effort using specific gears or eliminates fishing during certain times of the year could be effective.

Manuscript II: Sardinella stocks made up of flat sardinella (Sardinella maderensis) and round sardinella (Sardinella aurita) form the mainstay of the artisanal fisheries of Ghana. The landings of these stocks, however, have been dominated by small, immature fish in recent years. Yet, no comprehensive assessment has been conducted to identify the fishing gears that unsustainably harvest juvenile fish and what factors drive their distribution. In this study, we examined the distribution of sardinella species in the Ghanaian fishery and used length at first sexual maturity (Lm) of the fish as a reference point to determine the composition of the catches for each gear type and season. S. maderensis preferred coastal waters up to 65 m in depth, whereas S. aurita inhabited waters of depth up to 500 m. S. maderensis matured at 15.3 cm (± 0.013, N=1,777), which was greater than S. aurita at 14.2 cm (± 0.004, N=2,684). Recruitment of young fish into the sardinella stocks occurred throughout the year. Fishers used different gears to catch a wide variety of sizes of the two sardinella species at different depths and in different seasons. The average probability of capturing immature fish per each gear type was: beach seine (68%) > poli (46%) > set net (43%) > ali (22%) > watsa (4%). In other words, ali and watsa fishers were catching the majority of their fish above Lm. Fishers using poli, poli-watsa, beach seine and set net, however, were generally harvesting individuals below Lm suggesting that more juveniles are harvested by poli, poli-watsa, beach seine users in Ghana. These findings provide a baseline to encourage an increase in mesh sizes of poli, poli-watsa, beach seine and set net to a minimum size of 2.5 cm as a management tool to complement the ongoing seasonal closure in Ghana.

Manuscript III: The sardinella fishery forms the backbone of Ghana’s marine artisanal fisheries. There are two species in the sardinella fishery: flat sardinella (Sardinella maderensis) and round sardinella (Sardinella aurita). Catches of these fish, however, have been dwindling for the last decade due to overfishing. Yet, little research has been done to guide or inform the government of Ghana on possible outcomes from different fisheries management strategies. Hence, this study sought to explore two potential management solutions to achieve sustainable exploitation rates of the sardinella stocks. First, we determined the life-history characteristics of S. maderensis (7, 240 individuals) and of S. aurita (6, 848 individuals). We then simulated the population dynamics of both species under different seasonal fishing closures, with and without accompanying gear restrictions. The population growth rates of the two sardinella species ranged from 0.089 – 2.59 yr-1 under seasonal closures without gear restrictions, as well as under seasonal closures with gear restrictions. Growth rates, however, were relatively higher under the seasonal closures with gear restrictions as compared to the seasonal closures without gear restrictions due to high recruitment of juveniles into the fishery. Although longer seasonal closures resulted in higher population growth rates, we found that a one-month seasonal closure with gear restrictions to be the optimal management strategy. The gear restrictions would allow the fish to spawn at least once in their lifetime before they become harvestable. Under the current management status quo, our results suggest that both species will continue to dwindle and collapse within 15 – 20 years. These simulations provide managers with the tools necessary to forecast sardinella populations in Ghana under different policies, enabling sustainable management regulations to be established.

Download

Share

COinS
 
 

To view the content in your browser, please download Adobe Reader or, alternately,
you may Download the file to your hard drive.

NOTE: The latest versions of Adobe Reader do not support viewing PDF files within Firefox on Mac OS and if you are using a modern (Intel) Mac, there is no official plugin for viewing PDF files within the browser window.