Management issues: Fish Stocks & Habitats

The Kaipara has a long history of customary, commercial and recreational fishing and has also long been recognised as an important nursery area for juvenile fish such as snapper, mullet, flounder and rig. Anecdotal information and local knowledge also recognises the Kaipara Harbour as a nursery ground for mako, hammerhead and great white sharks. Kaipara hapû readily refer to the Kaipara as their ‘food basket’ and family member. The Kaipara holds tremendous cultural significance to Ngâti Whatua hapû Te Uri o Hau and Ngâti Whatua o Kaipara.

Most of the species caught in local fisheries are part of populations that range over a wider area than just the harbour. This is a complication for planning and management decision–making because it means that sizes of harbour populations are likely to be constantly changing due to natural fluctuations as well as fishing pressure. There is some information on the status of fish stocks in the Kaipara that has been obtained using a research technique called Catch Per Unit Effort (CPUE) – discussed further below.  

Management

(2) Demographic Changes

Demographic changes, or changes in target or non–target fish population characteristics (e.g. age, length), are the most obvious effects of fishing, especially for an overexploited population. While direct demographic changes have not been have not been scientifically quantified for many targeted fish stocks in the Kaipara, it is generally accepted that fishing: reduces the abundance of target species; changes the size and age structure of the populations through selective removal of larger, older individuals; and affects the spawning biomass and, with over–fishing, recruitment to the population (Botsford et al. 1997; Pitcher et al. 2000; Pitcher 2001; Pauly et al. 2002; Polunin 2002).

Historical and current evidence of this impact occurring has been observed and recorded, for example, for the West Coast North Island snapper (Pagrus auratus) SNA 8 stock in fish size and age compositions (Morrison et al. Submitted; Morrison et al. Unpubl. data)). Commercial catches in 1974/75 were composed of broad ranges in sizes of snapper, with 20−40% being over 20 years old and greater than 50% being between sizes of 40−60cm. Catches from 1988−90 contained few of these larger and older snapper, with the population now dominated by younger (4−8 years) and smaller (25−35cm) fish, with no evidence of population recovery as of the most recent comparable sampling in 2003/04.

Anecdotal evidence from local commercial flounder fishers also have noted the change in size of flounder being landed in the 30 years of fishing on the Harbour. They believe that the small mesh size of set nets used, is contributing to this demographic change in flounder populations within the Kaipara harbour.

(3) Bycatch

Bycatch is the catching of organisms that are not the primary fishing target. In most fisheries there is bycatch; including small individuals of the target species, or other species with little or no commercial value. The problem is widespread, and exaggerated by inefficient fishing practices, which may lead to high juvenile mortalities, and losses of non–commercial species. Additional issues can include reductions in fish quality (and hence economic returns), as from damage to target species in trawls with high levels of bycatch, and set netting when nets are left unattended and left to soak and fish overnight, or greater than 6 hours (the average netting time as tides turn) (KHSFMSG 2003).

(4) Spatial Conflict

Scarcity of some of the most sought after species has resulted in ongoing conflict between fishers themselves (KHSFMSG 2003, Peart 2007), recreational and customary fishers and also with fishers from other harbours and other non–fishing users of the harbour. Conflict exists over fishing method used and species targeted, such as trawlers operating outside the harbour along the coast applying heavy fishing effort to catch adult snapper. Harbour fishers have always believed trawling and Danish seining have significantly jeopardised the snapper population inside the harbour (Murton unpublished); an adult population being historically recorded as fished since Mâori arrived in the Kaipara. Thus, by targeted adult populations outside the harbour and subsequently damaging and modifying habitats, fishers believe trawlers affect the yield of other fishers who do not use such destructive gear.

• Competition within the fishing industry;
• Relationships between fisheries and other coastal industries, such as aquaculture, sand mining, energy and coastal development;
• Relationships between fisheries and charismatic megafauna (e.g. dolphins, orca, seals).

A point of discontent from local fishers has been the ability for non–local fleets to fish witin the Harbour. Because QMAs, for example flatfish (FLA1), cover large areas, when fishing has declined on the east coast, fishers with access to quota can trail their boats across to the West Coast Harbours and shift their fishing effort. This can severely impact on localised fishing operations, especially when fish stocks become depleted from extra effort from non–local fishing operators. This situation acts as a strong disincentive for any local efforts to manage the health of the harbour and fisheries.

Most of New Zealand’s biodiversity is in the sea (MacDiarmid 2007). Diversity within marine ecosystems is important for stable function and productivity (Kenchington 2003). When a species of commercial importance becomes extinct, or are reduced to low levels, harvest pressure is often transferred to others with similar traits, and reverberates through the levels of species assemblages and ecosystems.

Diversity is hypothesised to buffer ecosystems from the impacts of large–scale environmental changes. Thus, any reduction in diversity could affect an ecosystem’s ability to withstand change, and may instead see it undergo a major shift in trophic structure, composition and function. Worm et al. (2006) conducted a meta–analysis of published data over the past 35 years, across varying scales and ecosystems (coastal, estuarine, large marine ecosystems) and up to 80 economically and ecologically important species. The analysis examined the effects of variation in marine biodiversity (genetic and species richness) on primary and secondary producers, resource use, nutrient cycling and ecosystem stability. Across regional scales, it was found that with declining diversity, rates of resource collapse increased and recovery potential, stability, and water quality decreased exponentially. The analysis suggested that substantial loss of biodiversity is closely associated with regional loss of ecosystem services, notably filtering and detoxification services provided by suspension feeders, marine vegetation (seagrass) and wetlands.

As part of the analysis Worm et al. (2006) reported on the ability to reverse the trend of declining biodiversity. 44 fully protected no–take areas and four large–scale fisheries closures from across the World, showed a general trend of increased biodiversity and ecosystem services. Fisheries productivity (measured in CPUE) increased fourfold in fished areas around the no–take area.

Biodiversity is important from an ecosystem–based management point of view because it is related to “resilience” or capacity to resist an impact or return to original conditions after the impact is removed. As a consequence, it is of interest to fisheries that the diversity of exploited habitats and the diversity of habitats and species in them is maintained and possibly enhanced as an “insurance” against negative consequences of future changes.

With the removal of larger and older individuals, reproductive output is reduced because age–at–first and size–at–first maturity is smaller. Even though spawning stock biomass remains in its thousands, the effective genetic population size is only hundreds. To study and demonstrate the changes in genetic diversity of overexploited or collapsed stocks in wild populations is often complicated by the lack of suitable populations for comparison (Hauser et al. 2002). Most commercially exploited species are fished wherever they occur, and thus comparisons between exploited and unexploited stocks are not possible. For the New Zealand snapper fishery there is evidence of a loss of genetic diversity. The northern North Island stock of snapper was compared with the stock of Tasman Bay in the South Island which has not been exploited and heavily fished at the same level as the northern stock (Hauser et al 2002). It was found that commercial fishing had selective genetic changes in exploited stocks and also caused reduced genetic diversity by genetic drift.   Thus very few individuals contribute successfully to the next generation, possibly in part explaining the often poor relationship between the size of the spawning stock and recruitment. This is an area of new research. Collaboration between fisheries biologists and molecular geneticists is needed to protect individuals that are most likely to reproduce successfully from overexploitation.

• In conjunction with Regional and District Councils to assess whether stronger measures are needed to control run–off;
• To inform landowners – farmers and foresters with land in harbour and coastal catchments in particular of the potential effects of how they use their land on fisheries habitats and important fish species;
• To support community riparian area planting projects in view of their potential to reduce the quantity of sediment reaching streams and harbours.

Botsford, L. W., J. C. Castilla & C. H. Peterson (1997). The management of fisheries and marine ecosystems. Science 277: 509–514.

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