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SIODFA Fish Population Data Collection Programme

SIODFA vessels attempt to collect biological data during commercial fishing operations and now an extensive database exists of fish lengths, weights, sex composition and gonad stage, especially for orange roughy.  This programme is to be expanded to alfonsino.  An extensive collection of otoliths has also been collected for analysis and considerable progress has already been made in Australia thorugh government programmes on material collected by the F.V.s Southern Champion and Austral Leader.

Measurements from 100 fish are attempted when a trawl catch is taken.  Such information assists in understanding the population structure and population dynamics of the exploited fish stocks.  Guidance is given in:

Patchell, G.  Instructions for the collection of biological samples from orange roughy in the southern Indian Ocean.  51 – 59.  In Shotton, R. (Comp.) 2006.  Management of Demersal Fisheries Resources of the Southern Indian Ocean.   Report of the fourth and fifth Ad Hoc Meetings on Potential Management Initiatives of Deepwater  Fisheries Operators in the Southern Indian Ocean (Kameeldrift East, South Africa, 12 – 19 February 2006 and Albion, Petitie Revière, Mauritius 26-28 April 2006) including specification of benthic protected areas and a 2006 programme of fisheries research.  FAO Fish. Circ. 1020.  90pp

for the manner of measuring the variables to be recorded.

The following plots illustrate the nature of analyses that can be undertaken.

 

Figure 1

Relative lengths of samples separated by sex

 

Figures 2 and 3 provide box plots[1]of the lengths that give an indication of the spread of orange roughy lengths in each sample.

 

Figure 2

Box plots of length distributions - males

 

 

             

Figure 3 shows similar data for the female fish that were measured.  Two phenomena are evident: (a) the difference in means of the last sample, which were taken from a different feature and the greater spread on lengths of the male fish.

Figure 3

Box plots of length distributions – females

 

 

 

Study of female gonads can reveal interesting biological relationships.   The following table lists the mean gonad condition obtained from a number of samples.

 

Table 1

Mean Sample Lengths

 

 

 

 

Mean Gonad Condition

No. of gonads measured

1

03/07/2007

3.9

11

2

04/07/2007

3.75

4

3

05/07/2007

2.9

18

4

06/07/2007

3.4

16

6

08/07/2007

2.5

3

7

09/07/2007

2.7

13

8

10/07/2007

2.5

25

9

11/07/2007

3.5

20

10

12/07/2007

3.2

6

11

13/07/2007

4

3

12

15/07/2007

3

10

13

16/07/2007

2.1

14

14

01/08/2007

6

22

 

­It is evident that there is some form of relation between date of sampling and gonad condition as the following figure shows.

 

Mean Gonad Condition Plotted Against Date of Sample

 

If a scatter plot is prepared of all ovary and body weight data it is apparent that two functional relationships exist, (a) a steeper ovary-weight relation at lower body weight and a less steep relation at higher body weight.  These correspond to the samples collected on the Jimmy’s feature and those taken on 1 August.  These are illustrated in the following figures in which there is a transposition of the variables represented on the axes to better demonstrate the functional relationships.

 

 

 

 

 

 

 

Plot of ovary weight verses body weight – all data: this plot shows the two functional relationships between the variables

 

 

 

 

Plot of Body Verses Ovary Weight - Jimmy’s Feature Samples

 

The axes variables have been transposed: in reality ovary weight (here shown on the ordinate or Y-axis) would be a function on body weight (here shown on the abscissa or X-axis).

 

 

 

 

 

 

Following spawning a different picture is apparent.  Samples of 1 August 2007 show a much lower body weight, indicative of a post-spawning condition.

 

 

The linear regressions for these two situations are:

 

Jimmy’s Feature data:

 

 

1 August 2007:

 

.

 



[1]The rectangular area in a box plot presents the range covered by the central 50 percentile of observations, each tail, the upper and lower 25 percentiles.

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