Abstract
Limiting the harvest of Haliotis species by the use of shell length restrictions has been a common practice for many years. However these length restrictions are often based more on the need to protect a proportion of the population, rather than an analysis of best harvest practice. Increased emphasis on mÄtaitai reserves (closed to commercial fishers), and the use of individual transferable quotas allows increased specificity of the harvest through the use of localised regulations on both shell length and harvest rates.
Information is needed on the best harvest system for specific Haliotis populations to aid in maximising returns, whilst maintaining healthy populations. One promising method of exploring the effects of changing the shell harvest length regulations is via matrix population modelling. Matrix models can be used to explore the normal minimum shell length harvests, as well as a variety of slot type (e.g. 100-127 mm) harvests. This method of population modelling allows inclusion of the stock-recruit relationship and population growth rates, whilst negating the need for knowledge of population numbers.
Here I investigated a theoretical homogeneous midrange population based on H. iris measured at Kaikoura in 1968-70 to and the shell length restrictions and harvest rates that provided the highest sustainable annual harvest. Annual harvest was measured both in terms of numbers taken,and biomass yield. H. iris shell lengths can vary from 79 mm to 163 mm in different locations around New Zealand, and so a midrange population with an average maximum length of 146:2 mm was used. I found that a large slot type harvest system consistently maximised the numbers that could be sustainably harvested, however the total biomass yield was maximised from a minimum shell length longer than the current 125 mm, rejecting a trend towards the longer minimum shell lengths being introduced in many commercial H. iris catchments.
Several other results I have included are also of interest, besides the shell length recommendations. The preliminary calculation of a population growth rate of 16% for one region, based on Ministry of Primary Industry H. iris publications provides a starting point for the analysis of healthy Haliotis populations, however there were concerns about the accuracy of some baseline data. Due to uncertainty about the population growth rate values ranging from 0% to 16% were examined. Although absolute values of the elasticities and sensitivities were sensitive to changes in the population growth rates, their overall rankings remained the same. Although the optimal harvest lengths changed markedly at different population growth rates, the recommended proportion in the harvestable class remained constant. The suggested changes in harvest length had a mixed effect on harvester workload, with decreases in findability (proportion of adults in the harvestable class) often tied to increases in bodyweight (more biomass per animal harvested).
Distribution error in the matrix model was largely removed by the use of a spline function in 'R'. This was verified by multiple integrations based on equations containing exponentials and the construction of a set of consistently accurate matrices. These integration methods may also be useful outside matrix modelling. Consistently using just three classes created a manageable number of biologically relevant matrix elasticities which were then separated from elasticity measures influenced by the matrix construction. And finally the new terms of 'promotion' and 'relegation' were introduced to describe movement between matrix classes.
Many wild abalone fisheries around the world have severely declined or ceased, possibly due to poor management (Braje et al., 2009; Searcy-Bernal et al., 2010; Plaganyi et al., 2011), and research into specific shell harvest length is sadly lacking. The main aim of this research was to identify possible modelling methods that could be used to refine the setting of harvest length regulations. The importance of wild Haliotis populations economically, recreationally, biologically and culturally, both in New Zealand and internationally means further work in this area is important, and could lead to increases in sustainable yield whilst maintaining, or even increasing the long term stability of Haliotis populations.