The effects of prey density and duration of prey availability on the behaviour, growth and survival of larval Atlantic cod (Gadus morhua) and fat snook (Centropomus parallelus)

Temple, Shelby (2002) The effects of prey density and duration of prey availability on the behaviour, growth and survival of larval Atlantic cod (Gadus morhua) and fat snook (Centropomus parallelus). Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (11MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

The main bottleneck to the mass production of juvenile seed stock for the intensive aquacuiture of many marine finfish species is the high mortality associated with the larval stage. In an attempt to reduce the costs and increase larval survival for two species with aquaculture potential, experiments were performed to investigate ways at reducing the quantity of live food required. -- The first experiment described in this thesis was performed in Brazil with fat snook (Centrompomus parullclus) a species of commercial interest for which little is known about the natural history or culture techniques. Previous larviculture with this species has been done at prey densities of 30 000 prey litre. I tested a range from 30 000. to 5000 prey litre, and found no difference in survival and growth at an> of these prey densities. Reducing prey densities to only 5000 prey litre will result in considerable savings in labour and cost for future culture of this species. The behaviour of the larvae is also described here for the first time. Fat snook larvae were observed to be salutatory foragers, and to employ a sigmoid (s-curve) and lunge prior to capture, until the larvae reach lengths of 3.0 mm. -- The experiments with larval Atlantic cod (Gadus morhua) were more in depth since both the optimal prey density and foraging behaviour patterns were already known. The first experiment investigated the possibility of using a 'mismatch’ (low prey density) at one of three stages (endogenous, transition from endogenous to exogenous, and exogenous) in the larval period. It was found that larval survival was maximized when a low prey density (500 prey litre) was offered during the first 5 days post-hatch, and that growth and survival were maximized thereafter by offering a high prey density (4000 prey litre). The behavioural observations in this experiment did not concur with previous reports, which found that orient frequency increases with prey density. Comparisons of the methodologies between this and previous studies found that How rates differed. Flow rates were found to modify the duration of prey availability. A second cod experiment investigated the effect of decreased duration of prey availability (controlled by flow rate) and found that survival and growth decreased with decreasing duration of prey availability. The probable explanation for this decrease was that the larvae had less time to forage. Orient frequency of larvae that had less time to forage (high flow treatments) was 3 times higher immediately after feeding. Hourly behavioural observations demonstrated a sudden spike in orient frequency after feeding for larvae reared at high flow, which explains how larval cod are able to take advantage of prey patches that occur in their spatially and temporally variable environment. -- Results of the cod experiments can be used to suggest a new larviculture protocol, which schedules low prey density during the endogenous feeding stage and then increases to high prey density thereafter. The results also demonstrate that a relationship between flow rate and duration of prey availability exists and that larval cod are sensitive to the duration of time that is available for foraging. Future larval husbandry practices should include the recording of a prey clearance curve for each tank at set flows, so that the prey dynamics of each tank can be understood. Consideration should be given to both water quality and prey clearance rates in the assignment of flow rates.

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