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Ecological sustainability assessment for Firth of Thames shellfish aquaculture: Tasks 2-4: biological monitoring

TR 2005/06
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Report: TR05/06

Authors: N. Broekhuizen, J. Ren, J. Zeldis, S. Stevens (NIWA)

Abstract

NIWA were engaged by the Auckland Regional Council, Environment Waikato and the Western Firth Mussel Consortium to make quantitative predictions of the degree to which large-scale mussel farming in the western Firth would influence snapper egg / larval survival and plankton abundance and spatial distribution.

Three separate simulation models were used, and three distinct farm scenarios modelled (no farms (NF), existing farms (0), and existing farms plus a maximal modelled western Firth Aquaculture Management Area (AMA) (1)). For each farm scenario simulations were made under six distinct hydrodynamic conditions.

The simulations indicated that under the existing farm scenario, the Firth-wide numbers of snapper larvae surviving to age 8 days post-spawn may be reduced by 2-6 percent relative to the NF scenario, and by 2.5-15 percent under scenario 1.

Two biological models were used to examine the influence of mussel farming upon phytoplankton and zooplankton, a 'logistic' and a 'biophysical' model. The logistic model suggested that the concentrations of fast growing plankton will be reduced by less than 10 percent within the farms, and that vulnerable plankton will suffer depletion of >20 percent within the farm, with depletion halos extending several km beyond the farm. The biophysical model indicated that farms will tend to suppress total phytoplankton abundance during spring, and predicted depletion of 30 percent within the farm, extending several km downstream of the farm. The model also showed that phytoplankton enhancement may result from the farming activities.

The modelled western Firth AMA was predicted to have larger impacts than any of the existing farms, reflecting its larger size, the fact that it will occupy shallower water than most existing farms, and that current speeds are lower in this region that this AMA would occupy.

Ecological sustainability assessment for Firth of Thames shellfish aquaculture: Tasks 2-4: biological monitoring [PDF, 138 KB]

Contents
1 Executive Summary 1
2 Introduction 5
2.1 Comparison with earlier results 7
3 Methods 9
3.1 Weather/season scenario descriptions 9
3.2 Farm scenario descriptions 9
3.3 Farm details 11
3.3.1 Line arrangements 11
3.4 Model domain 12
3.5 The empirical model 14
3.6 The biophysical model 19
3.7 Mussel feeding sub-model 21
3.8 Implementing farms within the simulation models 22
3.9 Numerical solution 23
3.10 Simulations undertaken 23
4 Results 25
4.1 Interpretation of the plots 25
4.2 Snapper model 27
4.3 Logistic plankton 31
4.4 Biophysical model 37
5 Discussion 45
5.1 General comments 45
5.2 Over-estimation of mussel ingestion rates 46
5.3 Snapper model 47
5.4 Logistic plankton model 48
5.5 Biophysical model 50
6 Ecological implications 53
6.1 Snapper model 53
6.2 Plankton models 54
7 Conclusions and recommendations 57
8 References 59