Baseline study Fish, fry and commercial fishery Nysted Offshore ...

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Differences between length distributions of the different fish species from the two areas
(the wind farm and the reference area) were compared between studies and at different
months.
As the data for the distribution of fish length is often insufficient for a Two-way
analysis of variance to be used, a Kolmogorov-Smirnov Z test was used instead. The
Kolmogorov-Smirnov Z test is a non-parametric test for the difference in the
distribution of frequency, where time and place are fixed. This test does not allow the
interactions between time and place to be assessed.
To determine if mean lengths of the separate fish species are similar in both the wind
farm and the reference area a non-parametric Mann Whitney test was used. Length
distribution results and length frequency for each species is to be included in evaluation
of the non-parametric Mann Whitney test.
3.6.2. Power analysis
A power analysis was carried out after the statistical analysis to reveal the degree of
certainty achieved in the analysis, in other words the extent to which there is a
probability that the test will be able to show a real difference from a given amount of
data.
To run a power analysis at least three conditions that must be fulfilled:
1) The starting point is a particular method of statistical analysis, for example
the ANOVA test, in connection with the BACI design.
2) The variance structure of the data must be known, including the number of
replicates used.
3) The selection of a difference in values that the analysis must be able to
detect. This is normally called the size of effect. In this case, the size of
effect was selected as 50%.
The selection of the size of effect requires the balance of two contradictory factors. For
certain biological processes, smaller sizes of effect can be of relevance, but selecting a
smaller size of effect will mean a given study is less powerful. This creates a need for a
larger number of samples in planning the subsequent sampling using the actual power
analysis as the basis.
If a power analysis returns a power of 10% for a given statistical test with a size of
effect of 50%, results in only a 10% probability that the test will give a significant
result, even though there is a real difference in the data of 50% for the variable selected
between two studies of either time or space.
A power analysis can also be used to calculate how many replicates are required to
achieve the desired power (for example 80%) for a given test with a selected size of
effect (for example 50%).
Power analyses were used in this study for the dual variable analyses (BACI) for
determining the difference in the number and total weight of the various species of fish.
SEAS. Baseline study – Fish, fry and commercial fishery Dok. nr. 2148-03-001-rev3 2P.doc