The Catch

Fresh­wa­ter trou­bles

Boating NZ - - Contents - BY JOHN EICHELSHEIM

Catch­ing and eat­ing white­bait is as tra­di­tion­ally Kiwi as, well, pavlova, but all is not well with the world for this iconic New Zealand del­i­cacy. White­bait com­prises the fry (ju­ve­niles) of five na­tive fresh­wa­ter fish species, but our fresh­wa­ter fish, in­ver­te­brates and plants are in trou­ble. A re­cently-pub­lished re­port, Our fresh­wa­ter 2017, pre­pared for the Min­istry for the En­vi­ron­ment and Statis­tics NZ, found that of the 11 fresh­wa­ter fish species for which there is re­li­able data, six have de­clined in abun­dance in the last 39 years.

Of the na­tive species re­ported on, around three-quar­ters of fish, one-third of in­ver­te­brates, and one-third of plants are threat­ened with, or at risk of, ex­tinc­tion.

New Zealand has 53 known res­i­dent na­tive fresh­wa­ter fish species. In ad­di­tion, 21 fresh­wa­ter fish species, in­clud­ing trout and sal­mon, have been de­lib­er­ately or ac­ci­dently in­tro­duced to New Zealand. Rain­bow and brown trout are also in de­cline.

Many fish species mi­grate be­tween the sea and fresh wa­ter to com­plete their life cy­cles, mean­ing they can be se­verely af­fected by bar­ri­ers to mi­gra­tion in rivers and streams.

DE­CLIN­ING WA­TER QUAL­ITY

There are sev­eral rea­sons our fresh­wa­ter ecosys­tems are in de­cline. They in­clude changes to catch­ment land cover, land use, in-stream habi­tat, flow regimes, fish pas­sage (routes for mov­ing up and down rivers and streams) and the ef­fects of in­va­sive pest species (an­i­mals and plants) and pol­lu­tants.

The big­gest chal­lenge is rapidly de­clin­ing wa­ter qual­ity, due to much higher silt and nu­tri­ent loads. Our fresh (and coastal) wa­ters have been suf­fer­ing higher silt loads ever since hu­mans ar­rived.

We lose about 192 mil­lion tonnes of soil into water­ways and the ocean every year.

In 450-odd years, Maori burned al­most half of New Zealand’s orig­i­nal for­est cover to flush out game, clear land for gar­dens, and en­cour­age bracken fern, a valu­able food source. In the South Is­land, vir­tu­ally all the for­est in the Can­ter­bury Plains, Mckenzie Coun­try and many other re­gions was re­moved.

Later, when Euro­pean set­tlers be­gan con­vert­ing the coun­try’s re­main­ing bush­land into pas­ture, the vol­ume of top­soil washed off the land in­creased ex­po­nen­tially, which fun­da­men­tally changed streams, rivers, lakes, har­bours and es­tu­ar­ies, es­pe­cially around north­ern New Zealand.

Too much fine sed­i­ment can se­verely de­grade streambed habi­tat. It re­duces wa­ter clar­ity and has neg­a­tive ef­fects on streambed life. It has sim­i­larly dam­ag­ing ef­fects on lakes, es­tu­ar­ies and in­shore ma­rine habi­tats. Be­fore hu­man set­tle­ment, fine sed­i­ments are thought to have covered 8% of New Zealand’s riverbeds; in 2011 the fig­ure was 29%.

For ex­am­ple, be­fore for­est clear­ance the Kaipara Habour was clear, as were all the tree-lined wa­ter­courses emp­ty­ing into it, in­clud­ing the now dirty-brown North­ern Wairoa River. To­day it is usu­ally a shade of green-brown, though it is prob­a­bly clearer than it was 100 years ago when land clear­ance was in full swing. Start­ing in the 1850s, whole­sale for­est clear­ance and in­creas­ingly in­ten­sive land use mas­sively in­creased sed­i­ment loads in the rivers and streams emp­ty­ing into the Kaipara.

The same holds true for al­most every wa­ter­shed in New Zealand.

HEALTH MARK­ERS

In all but the most iso­lated, wild por­tions of the coun­try, ero­sion, soil leach­ing and pol­lu­tion/con­tam­i­na­tion from farm an­i­mals and hu­mans has de­graded New Zealand’s water­ways.

Hu­man ac­tiv­i­ties on land can cause ex­cess nu­tri­ents and Escherichia coli ( E.coli) to wash into wa­ter bod­ies through runoff, or fil­ter through the land into ground­wa­ter. Phos­pho­rus of­ten

The big­gest chal­lenge is rapidly de­clin­ing wa­ter qual­ity, due to much higher silt and nu­tri­ent loads.

en­ters sur­face wa­ter at­tached to sed­i­ment. Even to­day many of our rivers and lakes are repos­i­to­ries for in­dus­trial, agri­cul­tural and hu­man waste.

The Our fresh­wa­ter 2017 study fo­cusses on two nu­tri­ents, ni­tro­gen and phos­pho­rus, which tell sci­en­tists about the risks of al­gal blooms. It also looks at E.coli, an in­di­ca­tion of fae­cal con­tam­i­na­tion, which can in­di­cate whether wa­ter bod­ies are safe for re­cre­ation. Ris­ing E.coli lev­els in many of New Zealand’s rivers and lakes is at the heart of re­cent calls for ‘swimmable’ rivers.

Both the con­cen­tra­tion and the ra­tio of ni­tro­gen and phos­pho­rus in a wa­ter body are im­por­tant, as al­gae needs a sup­ply of both nu­tri­ents for ex­ces­sive growth (blooms) to oc­cur. Con­cen­tra­tions of ni­tro­gen and phos­pho­rus are much higher in ur­ban and pas­toral ar­eas than in na­tive ar­eas, so the like­li­hood of al­gal growth is higher in these en­vi­ron­ments.

In agri­cul­tural ar­eas, nu­tri­ents and pathogens (or­gan­isms that can cause dis­ease) come from an­i­mal waste and urine – and fer­tilis­ers. In the last 50 years agri­cul­tural prac­tices have in­ten­si­fied in some ar­eas of New Zealand, in­di­cated by higher stock­ing rates and yields, with com­men­su­rate in­creases in the use of fer­tiliser, pes­ti­cides and food stocks.

Per­haps the most sig­nif­i­cant trend has been the swing to­wards more in­ten­sive forms of agri­cul­ture, such as dairy­ing. Agri­cul­tural land use is the world’s great­est con­trib­u­tor to dif­fuse pol­lu­tion (run-off from the land or fil­tra­tion through the soil).

In ur­ban en­vi­ron­ments, con­tam­i­nants en­ter wa­ter bod­ies mainly through stormwa­ter and waste­water net­works, il­le­gal con­nec­tions to those net­works, and leaky pipes, pumps and con­nec­tions. New Zealand’s cities are grow­ing, in size and pop­u­la­tion.

In most of New Zealand, wa­ter qual­ity is not get­ting bet­ter. Ni­trate-ni­tro­gen con­cen­tra­tion in the rivers mon­i­tored was wors­en­ing in more cases than were im­prov­ing. Phos­pho­rous con­cen­tra­tion was im­prov­ing in more rivers than were get­ting worse, but some rivers showed de­clin­ing lev­els of ni­traten­i­tro­gen with in­creas­ing phos­pho­rous loads and vice versa.

CHANG­ING LAND USE

The re­cent shift from rel­a­tively low in­ten­sity pas­toral sheep farm­ing to in­ten­sive dairy­ing, of­ten sup­ported by ir­ri­gation, is mak­ing the sit­u­a­tion worse.

Ir­ri­gation ac­cel­er­ates grass pro­duc­tion, sup­port­ing higher stock­ing rates, but it also leaches nu­tri­ents from the soil more quickly, forc­ing farm­ers to ap­ply more fer­tiliser. At the same time, in­ten­sive stock­ing pro­duces more an­i­mal waste which ends up wash­ing into our water­ways.

Ir­ri­gation re­quires wa­ter ex­trac­tion, ei­ther from aquifers, which can lower the wa­ter ta­ble, or from rivers and streams, leav­ing less wa­ter to di­lute pol­lu­tants, es­pe­cially in dry/drought years. More than half the wa­ter al­lo­cated or con­sented by coun­cils around New Zealand is for ir­ri­gation.

Forestry is an­other in­dus­try re­spon­si­ble for de­grad­ing wa­ter qual­ity, through ac­cel­er­ated ero­sion dur­ing har­vest­ing op­er­a­tions, fer­tiliser run-off and the ef­fects of ‘slash’ find­ing its way into wa­ter­courses.

TAM­ING RIVERS

Phys­i­cal changes to our rivers, such as stop­banks and rede­fined river channels have im­proved flood con­trol and al­lowed us to ex­ploit flood­plains for ur­ban and agri­cul­tural de­vel­op­ment. How­ever, these changes can cause river banks to erode and more sed­i­ment to be de­posited down­stream. The struc­tures – dams, weirs, cul­verts – we place in rivers can also hin­der fish mi­gra­tion.

By dis­con­nect­ing rivers from their flood­plains (and as­so­ci­ated lakes and wet­lands), the ben­e­fi­cial ef­fects of flood pulses are lost. A flood pulse is the pe­ri­odic flood­ing of a river, which aids in dis­pers­ing seeds, es­tab­lish­ing plants, cy­cling nu­tri­ents, scour­ing, de­posit­ing sed­i­ments, and main­tain­ing the rich­ness of species. Chan­nelised rivers sel­dom in­un­date their flood plains.

Wet­lands fil­ter nu­tri­ents and sed­i­ment from wa­ter, ab­sorb flood­wa­ters, and pro­vide habi­tat for plants, fish and other an­i­mals. Most of New Zealand’s wet­lands have been drained, lead­ing to a loss of bio­di­ver­sity and nat­u­ral func­tion. Their de­struc­tion con­tin­ues: be­tween 2007 and 2014, South­land lost 1235 hectares of wet­lands, 10% of the dis­trict’s to­tal. Only 10% of New Zealand’s orig­i­nal wet­lands re­main.

Our fresh­wa­ter 2017 is a wake-up call. Right now, we risk los­ing not only white­bait and other valu­able na­tive fish species, but also a world-renowned trout fish­ery.

If the cur­rent trend of wors­en­ing wa­ter qual­ity is to be re­versed, we need to match fu­ture land use to to­pog­ra­phy and soil type. In­ten­sive dairy­ing in arid re­gions, for in­stance, or beef cat­tle on steep, ero­sion-prone hill­sides should be dis­cour­aged. Tougher reg­u­la­tion is in or­der, be­cause if we do noth­ing, soon we won’t be able to swim in any of our rivers and lakes. BNZ

ABOVE LEFT Gi­ant kokupu make up part of the species mix we call white­bait, so pop­u­lar when cooked as frit­ters (cen­tre). ABOVE White-bait­ing re­mains a pop­u­lar ac­tiv­ity around New Zealand, but is it sus­tain­able?

LEFT Very few rivers are now pris­tine, es­pe­cially those in ur­ban and agri­cul­tural re­gions.BELOW Pas­toral sheep farm­ing is rel­a­tively low in­ten­sity, but it takes ad­van­tage of his­toric land clear­ances.

Newspapers in English

Newspapers from New Zealand

© PressReader. All rights reserved.