McHUGH,j.L., 1985The estuarine ecosystemintegrated. Foreword. Chap. 2:9·16In: A. Yaiiez·Arancibia (Ed.) Fish Commu.nity Ecology in EstWJriesand Coastal Lagoons: Towards an Ecosystem Integration, 654p.DR (R) UNAM Press Mexico 1985, ISBN 968·837-618-3

2. THE ESTUARINE ECOSYSTEM INTEGRATED. FOREWORD

2. EL ECOSISTEMA ESTUARINO INTEGRADO. PROLOGO

JOHN L. MCHUGHMarine Sciences Research Center

State University of New York at Stony BrookLong Island, New York 11974

USA

ABSTRACT.RESUMEN. .. .2.1 INTRODUCTION2.2 AREAS OF CONCERN ...........•.

.9......9

..........10....10

2.3 CHARACTERISTICS OF LAGOONS AND ESTUARIES .... 132.4 INTEGRATION OF THE SYSTEM, FROM FRESH

WATER TO THE SEA. . 142.5 REFERENCES. . 14

•

ABSTRACT

Estuaries and coastal lagoons are highly productive environments which can produce large numbers of fishes andtransfer that energy to the open sea. They are also harsh environments, however, with sometimes rapidly changingcondjtions, which can produce highly variable numbers of fishes and shellfishes. As conditions change, fishesmigrate in response. moving up and down the estuary in response to changing salinity or dissolved oxygen, movingfrom shallow water to greater depths in response to changing temperature, or moving out to sea where conditionsare less variable. Under extreme conditions they can respond by developing abnormalities or disease, or whenconditions become intolerable and inescapable. may die.The generally-accepted definition of an estuary, as a semi-enclosed body of water which has a free connection withthe open sea, is too restrictive to have much meaning for most migratory fishery resources. In the middle Atlanticregion, for example, most species art' boreal, migrating south in winter, or warm-water fishes, which move northand inshore in summer. Thus, the-estuary and tht" open se-amust be integrated if we are to understand fully theenvironment in which fishes live. The principal charactt"ristic of estuaries from man's point of view is that they are apart of the coastal environment and thus are subject to pollution from domestic and industrial wastes, which mayunder extreme conditions affect the-ir high productivity.

RESUMEN

\

Los estuarios y lagunas costeras son medios ambientes altamente productivos que pueden producir gran numero depeces y transferir esa energia al mar. Ellos son tambien medios ambientes rigurosos; sin embargo, con esascondiciones que cambian rapidamente, pueden producir un numero de peces, CfusUlceos y moluscos altamentevariables. Como condici6n de cambios los peces responden migrando, moviendose bacia arriba 0 abajo en elestuario en respuesta a los cambios de salinidad y oxigeno disuelto, movi~ndose desde aguas someras hasta grandt'Sprofundidades en respuesta a los cambios de temperatura, 0 moviendose hacia el mar donde las condiciones sonmenos variables. Bajo condicion~ extremas ellos pueden responder desarrollando anormalidades 0 e-nferme-dades,o cuando las condiciones se hacen intolerables y no hay escape, pueden morir.La definici6n generalmente aceptada de un estuario, como un cuerpo de agua semief!~e.9'ado que tiene unaconexion libre con el mar abierto, es demasiado restringida para tener mayor significado para la mayona de losrecursos pesqueros migratorios. En la region media del Atlantico, por ejemplo, la mayona de las especies sonboreales, migrando al sur en inviemo, 0 peces calido temperados, los cuales se mueven hacia el norte y hacia aguasprotegidas en verano. Asi, el estuario y d mar abierto deben ser integrados si se pretende comprender cabalmenteel medioambiente en el cuallos peces viven. Las caracterfsticas principales de los estuarios desde el punto de vistadel hombre es que elias son una parte del medioambiente costero, y asi estoin sujetos a la contaminad6n pordeshechos domesticos e industriales, 10 cual bajo condiciones extremas puede afectar su alta productividad.

•

Comribucion 429 from th~ Marin~ Sci~nc!' R!'Search C~nc~r, Stat~ Univ!'rsity of N!'w York at Stony Brook,

10

ver extend even farther, having a noticeable effect on 5ali~nides as far north as central Vancouver Island, BritishColumbia, and as far south as northern California, and attimes a( least 250 miles seaward (Barnes et al. 1972).

Fishes are for the most part highly motile, and move inand out of estuaries and coastal lagoons treely. In fact, re-latively few fishes are entirely confined to the estuary. Tosome degree this movement is related to the ebb and flowof salinity, as it fluctuates with the tides, with rainfall,and melting snow _It is also Tela ted to changing require-ments of the fishes as they grow and move with the se-asons. An extreme example is the anadromuos Pacific sal-mon, which swims hundreds of miles up rivers to spawn,may spend one or more years in lakes before going to sea,then spends another year or more, sometimes hundreds ofmiles from its native river, before returning as a matureadult to spawn. The estuary is but a transitory experienceto such a fish, as it moves back and forth through it on itsway to spawn, or to feed and grow. As Horn and Allen(1985) have pointed out in this volume, fishes in estuariesare mainly juveniles that utilize the littoral zone as a nur-sery area.

Why then do we talk about fish community ecology inestuaries and coastal lagoons, as if there were somethingspecial about this environment? It is because there is so-mething speciaL These bodies of water are shallow, withluxurious rooted vegetation often growing around its bor-ders; they are semi-enclosed, and thus offer protectionagainst the most violent storms; they have variable tempe-ratures and salinities, and sometimes variable oxygencontent and other characteristics, and are considered tobe a harsh environment for that reason; they have predo-minantly muddy bottoms, high turbidity, even thoughthey are somewhat sheltered from major wave action; andtheir mean net primary productivity is among the highestof any major ecosystem in the world (Day and Yanez-Arancibia 1982). The few resident fishes are relativelysmall, and the transitory fishes are often larvae and juve-niles, so the bulk of the fishes are numerous but small. Inthese environments fishes act to: 1) transfonn energyfrom primary sources; 2) actively conduct energy throughthe food web; 3) exchange energy with neighboring ecosy-stems through exportation and importation; 4) constitutea fonn of energy storage within the ecosystem; and 5)function as agents of energetic regulation (Yanez-Arancibia and Nugent 1977).

2.1 INTRODUCTION

JOHN L. McHl(G~

The waters of the earth are part of a continuuw. -Theyfall from the sky as rain; collect in streams, lakes, and ri-vers and flow toward the sea; filter intO'the soil as ground-water, to come to the surface later; flow down the riversand enter estuaries and coastal lagoons; and finally spHIinto the sea, to circulate for cywhile, some of it evapora-ting to enter the atmosphere as water vapdr, forming intoclouds and precipitating to begin the cycle once again. Inthe course of this complex cycle they are relatively purein the atmosphere, pick up various waste products as theyfall through the air, pick up more wast~s and dissolvedsubstances as they flow over, or sink into the ground.These dissolved natural and waste products of civilizationare augmented as they flow toward the sea, reach a maxi-mum somewhere in the rivers or estuaries, and then arediluted again as they enter estuaries and the sea. Finally,the waters are cleansed one,eagain as they evaporate andbecome airbone.

The fishes of the earth exist in great variety. They arecold-blooded, typically have vertebral columns, gills, andfins, and are dependent on water for their existence.Other animals, like mammals, reptiles, and invertebra-tes, are also dependent upon water, but these for the mostpart are beyond the scope of this paper. Fishes are themost numerous of the vertebrates. Best estimates are thatabout 20,000 recent species presently exist, although so-me estimates range as high as 40,000. They in size fromsome halfbeaks and a dwarf goby of the Philippines,which breed at half an inch or less, to such giants as thewhale shark, which attains lengths of about 70 feet andweighs 25 tons or more. They live in Arctic and Antarcticwaters, which are below freezing, to hot springs at tempe-ratures above 100°F. They are,.,foundin pure fresh water,or at salinities at least as high as 80 percent. They arefound in torrential mountain streams, and in deep, dark,and still ocean basins. Their vertical range extends at le-ast ten miles, from three miles above sea level to nearly se-ven miles below it (Bond 1979).

Estuaries and coastal lagoons are typically described onthe basis of their land boundaries. For this reason, andbecause people are land animals, they tend to think of es-tuaries in relation to the land. Thus, Pritchard's defini-tion (1967): An estuary is a semi-enclosed coastal body ofwater which has .afree connection with the open sea andwithin which sea water is measurably diluted with freshwater denved from land drainage, has been generally ac-cepted. This is probably acceptable for the most part asfar as sessile animals are concerned, but it is not at allsuitable for fishes. For one thing, coastal waters outsideestuaries are often measurably diluted with fresh waterderived from land drainage_ For example, the watersfrom the Chesapeake Bay drainage system affect the sali-nity of coastal waters outside the bay also (Schubel et al.1977), extending at least as far south as Oregon inlet, N.C. after hurricane Agnes. The waters of the Columbia Ri-

2.2 AREAS OF CONCERN

The waters of the surface of the earth, for our purposes,consist of four primary categories. These are streams, la-kes, ,and rivers; estuaries and coastal lagoons; coastal wa-ters beyond the estuary; and the open sea. Each categorycontributes fishes to the other categories, in varyingdegrees depending on species and on the ever changingenvironment.

II

I

THE ESTUARINE ECOSYSTEM INTEGRATED

Musick (1972) listed 287 species of marine, freshwater,and estuarine fishes which had been reported to occurwithin the Chesapeake drainage basin, freshwater speciesthat apparently never enter salt water. Another 36 enterbrackish or salt water to varying degrees, the extremebeing 23.4 parts per thousand. Only eleven were strictlyestuarine species, remaining permanently' in waters ha-ving some salt, but never going into salinities higher thanabout 33 parts per thousand_ Another eleven wereanadromous or catadromous, that is, either spawning infresh water but spending most of thie lives in the sea, orspawning in the sea and spending an important part oftheir life in fresh water. The remainder, 188 in number,were marine. Eighty-seven of these are strictly marinespecies, entering the estuary only occasionally. Another70 were polyhaline to varying degrees, some able to enterbrackish water almost to fresh water, and another 20could withstand salinities anywhere from sea water tofresh water, although some can enter fresh water only inthe juvenile stages, requiring higher salinities as they growolder. Thus, every possible environment. is occupied byone or more fishes, but the pattern described by Remaneand ScWieper (1974) holds, with a relatively low speciesdiversity in intermediate salinities, but high environmen-tal diversity. Blaber (1985) in this volume, has describedthe fishes of East African estuaries in much the same way.If we examine the commercial fish catch of ChesapeakeBay we find that the number of important species is farless. In fact, 25 species made up 99.9 percent of theweight and 99.6 percent of the value of fishes landed andmarketed in 1979 (Table 1). Three species were freshwa-ter species that ranged up to 17.6_partsper thousand sali-~nity, five species were anadromous or catadromous, andseventeen were marine species that for the most part ran-ged from ocean salinities to fresh water at various stagesof their life history or were tolerant of quite lowsalinities.One major group in this category was made up of a num-ber of species, usually small or not popular as food, usedas bait, animal food, or for reduction. None was strictlyestuarine.

Estuarine species, for the most part, are small speciesthat remain for the most part close to shore, especially insummer, and serve as forage for many of the marine orfreshwater species that enter the estuary seasonally. Tlieyare thus valuable in the transfer of energy from' the __tmarsWands to fresh water and the open sea.

Several authors, including myself (McHugh 1916). ha-ve emp~ized the importance of estuaries to marinefisheries and fIshery resources by demonstrating that alargt>part of the total weight of landings around the co-asts of the United States, or in any other part of the worldfor that matter, is made up of species that spend impor-tant parts of their lives in estuarine waters. It is equaHyeasy, however, to make the argument that most of the\efishes also spend considerable parts of their lives in tHeopen ocean. Thus, whether fishes are "estuarine-dependent" or "ocean-dependent" may be largely a riJ.'a't-

II

TABl.E I

THE TWENTY-FIVE MOST IMPORTANT FISHES IN TERMS OF WEIGl-tT LANDED INCHESAPEAKE BAY IN 1979, WEIGHTS IN THOUSANDS OF POUNDS. AND LANDED

VAl.UES IN THOUSANDS OF DOl.LARS.

Sp..cills1967

Value Dollan

Atlantic ....nhad..n (B1'evoor-tia tymnnu.)

Unclassifi ..d bait, animal food, and ttash. fish

457,132

12,424

13.214 6,078

458 211

S1.UIlD .. r flounder (Paraliahthys <kntatutr)

Weakfish (Cynosci(1l'l r'e(JaU8)

Bluefish (Pomat()IfUS sattatri:c)

Spot (Leiolltomus =thul'Us)

Atlantic croak"r (MicropogOTt W'ldutatu,,)

Al..wives (.litosa pS8WiOhaJotm(JU8,II. aestivatis)

Black sea bass (C.mtroprilltis striata)

Catfish and bullh ..ads {IataZul'Us spp.}

ArM.rican"e.l (AII(JUitza 1'Olltroata)

Strip ..d bass Owrcme s=tiUs)

Ara"rican shad (Alo"a sapidissima)

Scup (Stomotomus chrysops)

Whit .. p.. rch (Mol'On.e<VII<'l"icana)

Goosefish (Lophius amemanusi

lIlu..fin tuna (ThunlluB thy"nus)

lIutterfish (P"prilus triacanthus)

Grayfish (SqtlQ.lus aaanthias)

Carp (Cypru,IIB aa~io) ...

Sllv ..r hak.. (Merluccius bit.inearill)

Winter flounder (Pseudopleul'O>1ect"s americallus)

Atlantic ma~k..r ..l (Saomber scOf1ibl'Us)

11,731

6,893

3,384

2,551

2,234

1,8n

1,562

1,'28

1,469

1,413

1,041

m

'"'"'"''"m'"'"no

"

5,139 2,364

1,545 711

352 162

51' 237

534 246

94 43

771 355

304 140

1,093 503

1,586 730

247 114

257 liB

231 106

174 80

197 91

90 41

n 10,29 13

23 11

"Other shouks

Unclassified fO<ldfish" ..

ter of definition, For example, in table 1, three speciesare freshwater species, although they range into the es-tuary to about half the salinity of sea water. The remain-der, with the possible exception of bluefin tuna, can allbe considered marine; fishes that are "estuarine-dependent" at some time'during their lives. However,using the other criterion., ali of the "estuarine dependent"species, and the bluefIn tuna, can be considered "ocean-dependent." Thus, the percentage of "ocean-dependent"species is slightly larger than that of "estuarine-dependent" species.

It might be argued that these figures are influenced toomuch by menhaden and other industrial species, whichmade up over half the total weight of fishes landed inChesapeake Bay in 1979. When these species are remo-ved, however, the additional groups added to make up 25categories of fishes are unclassified food fishes, which aremainly, or perhaps all, "estuarine dependent" by one cri-terion, and yellowtail flounder, which are "ocean depen-dent" by either criterion. Thus, no difference is made byeliminating industrial fishes. Eighty years or so ago,

, things might have been different. Intuitively, one wouldthink that coastal and anadromous species would have

_.madeup a larger part of the annual catch. As landings inthe Chesapeake Bay area for 1901 show, this is true(Table 2). American shad was about ten times as impor-

- tant and Atlantic croaker about twice as important then,whereas flounder was only about two percent of present

12 JOHN L. McHUGH

TABLE S.TABLE 2 •LANDINGS OF SHELLFISHES AND OTHER NOf\;·FISHES IN CHESAPEAKEBAYIN 1901AND

THE TWENTY,FIVE MOST IM"ORTANT SPECIES OF FISHES LANDED IN CHESAPF.AKE 1979 BYWEIGHT AND VALUE THOUSANDS OF POUNDS AND THOUSANDS OF DOLLARSBAY IN 1901 IN TERMS OF WEIGHT. WEIGHTS IN THOUSANDS OF POUNDS AND

LANDED VALUESIN THOUSANDS OF DOLLARS1901 1979--- Thousands Thousands 1967 ThOUSAnds Thous ..nds ""1967 Species of pounds of dollars dollars of pounds of dollau dollArs

Species Weight Value (DoHanAlnericanoyster 82, B72 5,653 19,360 21,685 26,983 12,412

Atlantic menhaden 280.616 445 1,524~- Blue crab 21,530 "7 1,394 66,652 14,426 6,636Alewives 27,660 206 705

lunerican shad 10,083 487 1.669 thIrd cl .... 1,872 '" '" ", 1,294 mWeak.fish 8,450 '" 531 Turtles ., , 7 '" "' '"Atlantic croaker 4,240 57 195 Frog. "Butterfish 1,531 41 140

J "TerrapinStriped bass 1,352 1<4 390

Catfish and bullheads 1,309Prawn

40 137

White perch 1,185 58 199 Shrllop

Bluefish 815 30 103 Surf clam 20,217 J 11.864 5,457

Spot 829 74 97 SeA Scallop 7,676 24,377 1l,213Spanish mackerel 573 44 m .' Oc"..n qu..t>og 6.644 1,768 mCrevalle 469 14 48 2,883 ~.294 2,435Soft cl ....Hickory shad 457 11 38Yellow perch Horseshoe

457 14 48 crab '" " "American eel 443 16 55 Squid "0 m "Sturgeon 314 29 99Conch m '" "C.rp 291 B 27

Flounders 260 Lobster " '" '"B 27

Mullet 226 6 " Jonah crAb '" "Pike and pickerel '00 8 77 Red crab "Pompano " B 27 Other >~

Suckers 63 , 3 ~hellfhh '" '" mSea bass 53 3 " GrAnd tot ..h 105,763 6,21~ 21.281 uS,sn 87.201 40,109Scup 33

day catches and black sea bass about the same. Yet thebalance was essentially the same, with estuarine depen-dence or ocean dependence about equal.Ifwe were also to include litheinvertebrates, then a dif-

ference would immediately become evident. In 1901(Table 3), all the invertebrates were estuarine dependent,'oysters, blue crabs, and hard clams. In 1979, however, se-veral important ocean-dependent invertebrates becameevident, surf clam, ocean quahog, sea scallop, and squid,and in total weight landed they represented roughly onequarter of all invertebrates. The difference is even moreimpressive if the value in standard dollars is used. Thus,the ocean, although in area it is much larger, and there-fore does not produce the some high productivity of mari-ne life, must not be ignored.

The crit~calpoint to recognize is that all parts of the en-vironment of a living resource are important. Thegenerally-accepted definition of an estuary, quotedearlier, is too restrictive to have much meaning for mostmigratory fishery resources (McHugh 1967).

Although estuaries generally produce large commercialand recreational catches, the zone from 3 to 200 milesproduces large catches also_As a rough example, in theNorth Atlantic region of the United States, the zone outto three miles from the coast in 1981 produced about 1.6

- No laodings reponed

* 0.5 thousands pound~ or leu

million metric tons of all fishery products to commercialfishermen, and about 0.4 million to recreational fisher-men; while the zone from 3 to 200 miles produced nearly1.1 million metric tons to domestic commercial fis-hermen, nearly 0.3 million to recreational fishermen,and about 77 thousand metric tons to foreign fIshermen_These figures are not strictly comparable, because the do-mestic commercial catch also includes fishes caught in la-kes and rivers. Thus, the offshore catch is not substan-tially different from the inshore catch, and this does notinclude the catch of tunas, which is largely in the offshorezone. Day et ai. (1981). presumably quoting McHugh(1967), noted that the commercial catch is five times asgreat in Chesapeake Bay as on comparable areas of thecontinental shelf, but this did not include foreign catchesoffshore. These have grown tremendously since 1967, andnorth of Chesapeake Bay, especially in the area of Geor-ges Bank, are especially large. It must also be pointed outthat the area of continental shelf off the North Atlanticcoast of the United States is considerably greater than thearea of the conventional estuaries. Day et al. (1981) alsopointed out that most marine fishes which migrate intoestuaries are juveniles of demersal feeders whose adultslive in shallow areas along the coast. They emphasize, as

THE ESTUARINE ECOSYSTEM INTEGRATED 13

do many other authors, that there are very few truly es-tuarine fishes. Thus, the estuarine ecosystem, although itis undoubtedly among the most productive zones on landor sea, exports much of that productivity in terms of lar-ger and older fishes to the open sea_The estuarine envi-ronment is harsh, with widely fluctuating conditions,which, although they may be favorable to young fishes attimes, which can take advantage of vegetation in shallowwater for protection from enemies and waves, and profitfrom the tremendous production of small food, are not asfavorable to larger fishes, or to young if conditions are un-favorable.

2.3 CHARACTERISTICS OF'LAGOONSAND ESTUARIES

The principal characteristics oflagoons and estuaries thatset them apart from other bodies of water are their va-riable salinity, which ranges from zero to as high as 80parts per thousand or more; their temperature, which ismore variable, and follows the short-term temperaturechanges of the atmosphere much more closely, than theopen sea. Turbidity is high and variable, usually reachinghighs during floods in the rainy season, when ice andsnow are melting in the 'watershed, or when high windsroil the water. Nutrient cycles are similar to those ofshallow coastal waters, but are reinforced by the influen-ce of land drainage, which thus is more inmediately ap-parent. The concentration of nutrients, therefore, is rela-tively high_Another characteristic of estuaries that is im-portant and must not be forgotten is the tide, which, de-pending on the location and character of the estuary,contributes greatly to the ebb and flow the waters, and byvaring the salinity regularly at any point in the estuary,affects the movements of the fishes, as Weinstein (1985)has shown in this volume.

Dissolved oxygen is a problem in some estuaries, as italso is in the open sea in some cases. In Chesapeake Baydissolved oxygen becomes reduced in the deeper watersbelow the halocline regularly in summer, as the highbiological productivity drains oxygen from the water mo-re rapidly than it can be replaced by diffusion and advec-tion. In late summer the deeper waters in the middle ofthe Bay may be completely devoid of oxygen. The condi-tion extends up some of the tributaries, also, especiallywhen hurricanes with heavy rains pass over the area,further stabilizing the water column. Many marine orga-nisms are killed when such conditions prevail for anylength of time (McHugh 1967, Davis and Laird 1977). Itis not certain that these include significant numbers offishes, Ikcause fishes can sense and avoid these areas.Their reaction to the extreme conditions created byhurricane Agnes apparently was to be displaced down ri-ver a distance of eight to fourteen miles (Hoagman andWilson 1977). Probably only if they become trapped byunusual circumstances, and cannot escape, do fishes suf-fer significant mortalities.

This is a natural phenomenon, which occurs regularlyeven in the absence of domestic or industrial pollution,although pollution most often comes to mind when suchevents occur and kill important commercial or recre-ational organisms. If additional oxygen-demanding subs-tances are added to the water, they will aggravate thecondition, and the frequency or severity of occurrencesmay increase. Hypoxic, and occasionally anoxic bottomwaters also have been observed in the open ocean off theNew Jersey coast (Sindermann and Swanson 1979), andoff Lousiana and Texas in the Gulf of Mexico (Boesch1983). Here also, large numbers of sessile organisms likesurf clams (Spisula solidissima), or in the Gulf, larval whi-te shrimp (Penaeus setiferus), may be killed. The extentand intensity of hypoxia appears to be related to the volu-me of freshwater discharge from the Hudson or Mississip-pi Rivers. It is inevitable that some people will attempt toplace all the blame on the load of pollutants from these ri-vers, although deaths occur infrequently, and of course itis possible that the frequency or intensity of kills may beincreased somewhat by pollution. This has not been pro-ven, however, and at pres~nt it must be regarded ashighly speculative.

The flora in estuaries and coastal lagoons are reduced innumber of species, but attached flora are often veryabundant, for example, marsWand species like Spartina,Salicornia, and Scirpus, and true estuarine species likeeelgrass, Zostera marina, in temperate waters. The floraand fauna generally have a high level of evolutionaryadaptation to stress, and have originated from marine,

~reshwater, and terrestrial sources. These characteristicshave been well discussed, although briefly, by Emery andStevenson (1957) and Hedgpeth (1957). As the attachedvegetation dies and disintegrates, it creates a rich substra-te for small organism~, and in its living form providesabundant shelter for small fishes. Again, Weinstein(1985) has pointed this out in this volume.

The high biological productivity of estuarine waterswould certainly be much less if there were not a two-directional exchange of water between the fresh watersand the sea. The flow of fresh water from the rivers runsout at the surface, and pulls in an equal amount of saltwater from the sea at greater depths. If the freshwaterflow increases, from heavy rains or melting snow, the flowof salt water up the estuary increases also. Between thesetwo layers of differing density there liesa surface of no netmotion, acrosswhich, vertically, there is a new flowof saltupward, and this tends to stabilize the salinity at anypoint in the system. But the tides play an important partin this exchange, also. Thus, the system is highly dyna-mic, with salinities changing daily, and at shorter andlonger time intervals. On the average, the salinity at anypoint is highest at high tide and lowest at low tide, butwinds may alter this relationship, and so may the flow offresh water. Temperature is almost equally variable,although somewhat more regular, varying day and nightin response to air temperature, and fluctuating in respon-se to winds and currents.

14 JOHN L. McHUGH

2.4 INTEGRATION OF THE SYSTEM, FROMFRESH WATER TO THE SEA

It appears, then, that estuaries and coastal lagoons havetwo characteristics that set them apart from the total ran-ge of salinity regim~s: they are highly productive, andthus can support large numbers of fishes; but they arealso harsh environments, that vary greatly and ratherquickly in temperature;salinity, dissolved oxygen. windstirring of bottom sediments, and other factors that areadverse for fishes. Thus, although they do produce largenumbers of fishes when conditions are ideal, their pro-ductivity may vary greatly if prolonged adverse conditionsprevail. They support in large numbers relatively few spe-cies of fish, and these are mostly young fishes, which, asthey grow, move down the rivers and down the estuary to-ward the sea. As they grow and become more active theyrequire deeper water in which to maneuver, and their ha-bits change1:also. As winter approaches they have tomigrate to deeper water to avoid temperature changes,and to get a more reliable supply of food. In tropical es-tuaries, where temperature is not.such a problem, salinitymay still change rapidly, and dissolved oxygen may be agreater problem because temperature is high.

Thus, the system has a continuity, which is only par-tially constrained by the salinity, temperature, and otherrequirements of individual speCies of fish. Some speciesare strictly freshwater and cannot tolerate apreCiable sali-nity, some can penetrate the estuary partially but cannottolerate full ocean salinity, a few are typically estuarine,and remain within a variable range of intermediate salini-ties, but they are relatively few, and others are typicallyoceanic fishes that can tolerate varying degrees of lowered

salinity, or must remain fairly close to oceanic water. Afew fIshes, mostly anadromous speCies, can tolerate anysalinity, and live a complicated life-history ·that carriesthem from fresh water to the sea and back again duringtheir life cycle. A few typically marine speCies can toleratefresh water during the early stages, but must migrate tothe sea as they grow. These various habits have evolvedover thousands or millions of years, as species have deve-loped and multiplied. We do not know all the reasons fortheir diversity, and probably never will, but it is fairly cle-ar that shelter for young, and' room for adults to moveand seek favorable temperatures, were important consi-derations.

The marine fishes of Delaware, for example (McHugh1981), are mainly made up of migratory species of borealor wann-water origin, which migrate seasonally eitherfrom the north or from the south, and also inshore andoffshore with the seasons. Musick et al. (1985) have alsonoted the same thing in this volume.

This volume treats specifically the ecology of fishes inestuaries and coastal lagoons. It dealS' with their taxono-mic composition, their patterns of diversity, distribution,abundance, their migrations and their reproductive and I

feeding strategies. It shows that estuarine waters are veryimportant to Mexico, forming a large part of the coastalarea, and that it is imperative to preserve these waters asfar as far as possible in their natural state. At the sametime, estuarine waters are only a part of the entire wateryworld, and many fishes move freely from one part toanother. Thus, it is important that all parts of the aquaticenvironment be considered and protected if we are tocontinue to harvest optimum sustainable yields.

•2.5 REFERENCES

BARNES, C. A., A. C. DuXBURY and B. A. MORSE 1972. Circulationand selected propertil"'s of the Columbia River effluent at sea, p. 41-80. In: A. T. Pruter and D. L. Alverson (Eds.). The Columbia Ri-ver Estuary and Adjacent Coastal Waters. Bioenvironmental Stu-dies. Univ. Washington Press, Seattle, Washington.

BLABER, S. J. M. 1985. The ecology of fishes of estuaries and lagoonsof the Indo·Pacific with particular refl"'rl"'nce to South EaSt Africa,Chap. 12. In: A. YAiiez·Arancibia (Ed.) Fish Community Ecologyin Estuaries and Coastal Lagoons: Towards an Ecosystem Integra-tion. Universidad Nacional Autonoma de Mexico Press, 654 p.

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