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In New Zealand about 85% of the population lives within 10 km of a beach where swimming, surfing, sunbathing, sailing, snorkelling, fishing and shellfish gathering are all very popular recreational pass times.
Most of these activities can still be enjoyed all around New Zealand but close to our towns and cities it is steadily becoming more difficult to participate in fishing and shellfish gathering because the numbers of these animals have declined so severely. In addition some beaches that used to be clean and firm ~ ideal for walking, jogging and beach sports ~ are now spoilt by the settlement of fine muddy sediments washed from the adjacent land.
This has occurred wherever the natural cover of protective vegetation (bush and forest) has been removed from the land for agricultural, commercial, industrial or residential property development. Where good forest cover remains, the fall of heavy rain to the ground is delayed by the thick canopy of foliage and on the ground below, the fibrous mat of decaying leaves, twigs and fallen flowers act as a sponge to absorb the rainfall and slows its run off.
When forest cover is removed and the ground opened up, falling rain is able to run away quickly. Rushing torrents scour away the top soil and wash it down to the sea with much of it running out through harbours and estuaries. In these wave sheltered sites the sediment washed from the land is able to settle across the broad flat shores and firm sand flats become covered with mud.
The sand flats eventually deteriorate into mud flats which are difficult to walk across and where some animals that rely on cleaner conditions can no longer live. Many shellfish and crustaceans become excluded either because they cannot easily remain at their preferred burial depth in the soft mud or because muddied water clogs their gills making respiration and feeding too difficult.
Wherever good beds of shellfish have not been stressed by excessive amounts of fine muddy sediments washed from the land, they have come under increasing harvesting pressure from the growing human population. As more new immigrants come to settle in New Zealand and, at the same time, the population shift throughout the country from rural areas into the towns and cities is increasing, the pressures on marine resources around urban centres are also growing.
Monitoring the state of fish and shellfish populations all around New Zealand is important to identify if they are stable and only being harvested in sustainable quantities or whether they are in decline. As there are too many shores for central and local government organisations to monitor, the involvement of schools and community groups, such as CCET, is extremely valuable. However, the data collected is only useful if it is collected in a disciplined manner and using the same or a closely comparable method to that employed for other shores elsewhere.
The procedures and equipment described here have been tested and refined for use by school age students, tertiary students and community groups. It is hoped that as more shores become monitored regularly, the standard methods of sampling and recording information will enable comparisons to be made between the ecological condition of many different shores around the country. This will allow sensible management programmes to be designed so that our popular shorelines remain in good condition and their sea food resources can be maintained in harvestable quantities into the future.
The health of a sea shore is indicated by the abundance and diversity of its marine life. The biodiversity (= variety of different animal and plant types) comprises representatives from different groups. The most obvious are shellfish (molluscs), shore birds and seaweeds but the many species of worms and crustaceans, along with smaller numbers of echinoderms, anemones, sponges, sea squirts and other more obscure groups are all important members of the complex ecology of seashores.
Some worms are active scavengers and predators, roaming the shore for carrion or prey but most live in small tubes buried in the sand and strain plankton from the seawater or sort through sediment for nutritious morsels.
Crustaceans include shrimps, prawns, crabs, barnacles, sea lice and sand hoppers whose feeding methods range from browsing on seaweeds, straining plankton, scavenging, predation and sorting sediment.
Echinoderms include starfish, cushion stars, urchins and sea cucumbers.
Molluscs are mostly bivalves (such as oysters, mussels and clams like cockles, pipi and tuatua that are all filter feeders straining plankton from seawater) or snails (univalves) such as grazing limpets, topshells and cat’s eyes, sediment sorting hornshells and carnivorous whelks.
Although they are numerous, worms and crustaceans are quite delicate animals that are easily damaged during sampling and are therefore very difficult to work with. Echinoderms are frequently tougher but seldom sufficiently numerous to be practical animals for routine sampling. The shells of molluscs make them robust and many species are extremely numerous so they are ideal for sampling programmes. Some species, such as cockles, pipi and tuatua, are of particular interest because they are the ones that are most commonly gathered for food.
Some sampling programmes have concentrated solely on the abundance of one or two species that are regularly eaten such as the cockle and pipi on harbour shores. The results from these surveys can be misleading because it is difficult to know if any recorded declines are the result of over-harvesting or due to some other changes in the environment.
Frequently the management response to declining numbers of harvested shellfish has been to reduce the daily allowable take per person or to ban collecting completely but this has frequently not resulted in the recovery of the shellfish beds. In recent years the daily take in the greater Auckland region has been reduced from 150 per person per day to 50 but there is no evidence that this reduction has stopped the decline on some shores.
Along Auckland’s and Northland’s west coast where the toheroa, a large and delicious clam, was once abundant, the length of the open season and the daily allowable take over the years was steadily reduced as toheroa numbers declined. The last open season in 1980 was for just one day when the allowable take was just 5 per person.
After over a quarter of a century of harvesting being prohibited there has been no substantial recovery of the toheroa beds and this indicates that it was never over harvesting alone that was responsible for the collapse in numbers of this shellfish. It is likely that a number of other factors have had environmental effects that have affected the shellfish populations. These may be associated with agricultural and forestry development or management of coastal land adjacent to the beaches or with other fisheries practices along the coast.
To determine if harvesting alone or a combination of harvesting and environmental changes are responsible for shell fish declines, it is necessary to monitor not only the target species (such as the cockle) but also the companion species that live alongside the cockle that are seldom or never harvested.
Cockles living in harbour flats are routinely closely associated with other bivalves. These include large numbers of small nutshells, smaller numbers of wedgeshells and sometimes, young pipi and harbour troughshells.
Several snails are also commonly associated with cockles. Harbour topshells graze algal films from the tops of the cockle shells and hornshells sort through nutritious surface sediments above or alongside the beds of shallowly buried cockles. Scavenging whelks are always present too, lying in wait just below the surface to track down and devour any cockles or other marine life that become sick or too weak to defend themselves.
People who gather cockles almost never collect any of the other animals apart from the pipi. Indeed most people who collect cockles are unaware that there are large numbers of nutshells living mixed up with the cockles. This is because fully grown nutshells are generally only about 6 to 8 mm long and very seldom greater than 10 mm long.
Some people might consider such small bivalves to be unimportant but their abundance and the numbers of other bivalves and snails indicate whether it is over harvesting or changes to the environment that are related to any decline in the abundance of cockles.
Four scenarios must be considered before conclusions can be drawn about declining numbers of the population of a particular shellfish such as the cockle:
1. If only cockles are sampled, counted and measured in a survey and it is found that the population size has decreased, then there is no way of knowing if the decrease is due to over-harvesting or environmental changes or both.
2. If cockles and companion species of shellfish are sampled, and only the abundance of the cockles have declined then there is clear evidence that the cockle decline is related to over-harvesting because the companion species’ numbers have not changed.
3. If cockles and their companion species of shellfish are sampled and the abundance of all of them have declined by similar amounts then there is a clear indication that changes to the cockle abundance have been caused by environmental changes and not by over harvesting. The decrease in the numbers of companion species can only have been caused by a deterioration in environmental conditions because they were never harvested.
4. If the abundance of cockles and their companion species have both declined but the proportional decrease in cockle numbers is greater than the other shellfish then the change to the cockle abundance may be due to both over harvesting and a deterioration in the environmental conditions.
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