Exercises 01 - M. Afonso-Dias
-------------------------------------------------------------------------------------------------------------UNIVERSITY OF ALGARVE
Faculty of Sea and Environmental Sciences
MSc in Marine Biology
FISHERIES BIOLOGY
Exercises no. 1
(1st Year-1st Semester)
2007 / 2008
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SAMPLING LENGTH AND WEIGHT
STATISTICAL ANALYSIS OF SAMPLE DATA
For this practical class you will have a random sample1 of sardine (Sardina pilchardus2)
collected from a purse seiner3 catch landed in Olhão fishing harbour (Algarve) on the 3rd
January 2008. The purse seiner catch of sardine was estimated in 5 tonnes.
Objectives:
1. To obtain the length composition4 of the sardine sample and to calculate the mean
weights by length class (1/2 cm below);
2. To analyse the sample data by producing a table to summarize the main sample
statistics and to plot the sample length composition.
3. To estimate the mean length as well as the mean weight in the purse seiner catch.
I – Laboratory work: sampling length and mean weight by length class
Considering the sample of sardine available:
1. Measure each individual total length5 to the half centimetre below in order to obtain the
sample length distribution (classes of ½ cm);
2. Record the number of individuals measured by length class (length frequencies) in the
sampling sheet;
3. Obtain the total weight of each length class and record it in the sampling sheet;
4. Calculate the total weight of the sample by adding all length class weights.
II – Statistical analysis of the sample data (grouped data)
5. Build a length frequency distribution table;
6. Draw a histogram6 of the relative length frequencies and calculate the following statistics:
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Random sampling > http://www.fao.org/docrep/009/a0198e/A0198E05.htm#ch3
Sardina pilchardus FAO Fact Sheet > http://www.fao.org/fishery/species/2910
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Purse seine nets FAO Fact Sheet > http://www.fao.org/fishery/geartype/249
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Length composition = length frequency distribution
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Measuring total length > http://www.boatusangler.com/images/FishMeasure.jpg
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Histogram > see, for example, http://www.itl.nist.gov/div898/handbook/eda/section3/histogra.htm
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Exercises 01 - M. Afonso-Dias
a)- range ( llast +1 − l1 ), where llast+1 is the upper limit of the last length class and l1 is
the lower limit of the first length class of the distribution;
b)- modal class l 0
c)- median class l50
d)- mean length l
e)- variance, s 2
f)- standard deviation, s
s
s2
or
n
n
h)- coefficient of variation, cv , in percentage
g)- standard error,
7. Calculate the individual mean weight, wi in each length class, li ;
III – Estimation of mean length and mean weight in the catch (inference).
8. Estimate, at a 95% confidence level, the sardine mean length in catch, L , using the
random sample obtained (i.e., estimate the 95% confidence intervals for L );
9. Calculate the sardine mean weight in the catch, W .
Recommended literature:
1. Cadima, E.L., 1980. Sampling and statistical inference. In: Selected lectures from the
CIDA/FAO/CECAF seminar on fishery resource evaluation. Casablanca, Morocco, 6-24
March 1978: Rome, FAO Canada Funds-in-Trust, FAO/TF/INT (c) 180 Suppl., pp: 57-64
(photocopied article)
2. Cadima, E.L., A.M. Caramelo, M. Afonso-Dias, P. Barros, M. Tandstad & I. de
Leiva-Moreno. 2006. Sampling Methods Applied to Fisheries Science: a Manual. FAO
Fisheries Technical Paper 434, Rome, FAO, 96 p.
http://www.fao.org/docrep/009/a0198e/a0198e00.htm
3. Brown, M.L. & D.J. Austen, 1996. Data management and statistical techniques. In:
Murphy, B.R & D.W. Willis (eds). Fisheries techniques. 2nd edition. American Fish Society,
Maryland, pp: 17-62
4. Holden, M. J. & D. F. S. Raitt, 1974. Manual of Fisheries Science. Part 2 - Methods of
Resource Investigation and their Application. (Capítulo 3). Disponível online em:
http://www.fao.org/DOCREP/003/F0752E/F0752E00.HTM
Spanish version: Manual de Ciencia Pesquera. Parte 2 - Métodos para Investigar los
Recursos y su Aplicación. (Capítulo 3). Disponível online em:
http://www.fao.org/DOCREP/003/F0752S/F0752S00.HTM
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Exercises 01 - M. Afonso-Dias
French version: Manuel de Science Halieutique. Deuxiéme partie - Méthodes de
Recherches sur les Ressources et leur Application. Doc. Tech. FAO Pêches (115) Rev. 1:
223p. (Capítulo 3). Disponível online em:
http://www.fao.org/DOCREP/003/F0752F/F0752F00.HTM
5. Sparre, P. & S. C. Venema, 1992 - Introduction to tropical fish stock assessment. Part 1.
Manual. FAO Fisheries Technical Paper Nº 306.1. Rev. 1. Rome, FAO, 376 p. Também
disponível online em: http://www.fao.org/docrep/W5449E/w5449e00.htm (Capítulo 2).
Sparre, P. & S. C. Venema, 1992 - Introduction to tropical fish stock assessment. Part 2.
Exercises. FAO Fisheries Technical Paper Nº 306.2. Rev. 1. Rome, FAO, 94 p. Também
disponível online em: http://www.fao.org/docrep/W5448E/W5448E00.htm (Capítulo 17).
6. Statistical books (sugestion, only):
- Zar, J.H., 1999. Biostatistical analysis. 4th edition. Prentice-Hall Inc, New Jersey, 929 p.
Statistical books available online:
- Engineering Statistical Handbook http://www.itl.nist.gov/div898/handbook/dtoc.htm#Top
- Statistica Manual http:// www.statsoftinc.com/textbook/stathome.html
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