Ao.a.rrrs, J. R. 1969. Ecological investigations aroundsome thermal power stations in California tidalwaters. Chesapeake Sci. 10: 145-154.

Bnnrr, J. R. 1944. Some lethal temperature relationsof Algonquin Park fishes. Univ. Toronto Stud.Biol. Ser. 52; Publ. Ont. Fish. Res. Lab. No. 63.49 p.

1952. Temperature tolerance in young Paciflcsalmon, genas Oncorhynchus. J, Fish. Res. Bd,Canada 9:265-323.

Bnnrr, J. R., lNo D. A. Hrccs. 1970. Effect of tem-perature on the rate of gastric digestion in fingerlingsockeye salmon, Oncorhynchus nerka, J. Fish. Res.Bd. Canada 27 ; 1767-1779.

Bnnrr, J. R., M. Hor-r,nNos, AND D. F. Alopnorcn.1958. The etrect of temperature on the cruising

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speed of young sockeye and coho salmon. J. Fish.Res. Bd. Canada 15: 587-605.

Fnv. F. E. J. 1,947, Etrects of the environment onanimal activity. Univ. Toronto Stud. Biol. Ser. 55;Publ. Ont. Fish. Res. Lab. No. 68. 62 p.

1957. The aquatic respiration of fish, p. 1-63.In M. E. Brown [ed.] The physiology of fishes,Vol. 1, Academic Press Inc., New York, N.Y.

Sar-o, E. O. 1969. Some environmental factors to beconsidered in the design of thermal power plants inthe Northwest. Trend in Eng. 2l (4): 3-7,32.

SNnoncon, G. W., aNo W. G. CocnnaN. 1967. Sta-tistical methods. 6th ed. Iowa State Univ. Press.Ames, Iowa, 593 p.

Vnnmrnc,F. J.,aruW.B.VERNBERG. 1969. Thermalinfluence on invertebrate respiration. ChesapeakeSci. 10: 23+240.

The Relation Between Lobster Abundanceo Sea Urchins,and KeIp Bedsl'z

K. H. MaNN

Fisheries Research Board of CanadnMarine Ecology Laboratory, Bedford Institute, Dartmouth, N.S.

aNo P. A. BnrnN

Department of BiologyDalhousie University, Halifax, N.S.

M.nNr, K. H., eNo P. A. BnrnN. 1972, The relation between lobster abundance' sea ur-chins, and kelp beds. J. Fish. Res. Bd. Canada 29: 603-609.

When subtidal communities are disturbed and sea urchin populations expand, theyfrequently overgraze their food supply, eliminating large seaweeds from considerable areas.The hypothesis is advanced that the lobster is a key species, controlling sea urchin popula-tions in eastern Canada, and that reduction of lobster populations below a critical densityhas led to overgrazing of seaweeds in many places.

MaNN, K. H., ANn P. A. BnnrN. 1972. The relation between lobster abundance, sea ur-chins, and kelp beds. J. Fish. Res. Bd. Canada 29:603-609.

Lorsque les communaut6s sub-cotidales sont d6rangdes et que les populations d'oursinsse d6veloppent, celles-ci d6nudent souvent, par excds de broutage, des 6tendues considdrablesde varechs, leur principale nourriture. Les auteurs 6mettent l'hypothdse que le homard joueunrOle-clefdans le contr6le des populations d'oursins de I'est du Catada, et que la diminutiondes populations de homards au-deld d'une densit6 critique a favoris6 le broutage excessif desvarechs en plusieurs endroits.

Received November 30. 1971

WnrN one considers the management of a com-mercially important species in terms of its trophic

lContribution to the International Biological Pro-gramme CCIBP 128.

2Bedford Institute Contribution.

Printed in Canada (J2357)

system, the conclusions are often different fromthose obtained when considering only the popula-tion dynamics of the one species. Although theevidence is as yet incomplete, we draw attentionto the possibility that heavy exploitation of lobstersmay lead to the overgrazing of seaweed by urchins,with resultant loss of primary productivity.J.

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Sea urchins and kelp-In St. Margaret's Bay, N.S.,Laminaria spp. are the most important subtidal algaein terms of biomass (Mann 1972). However, within theLaminaria zone there are various-sized patches of almostbare rock that are characterized by high densities ofsea urchins (Strongylocentrotus droebachiensls). Theaverage biomass of sea urchins in bare patches is roughly1200 glmz, while in kelp forests it is only 150 glmz(Mann unpublished data). Himmelman (MS 1969)observed an inverse correlation between sea urchin andkelp abundance in Newfoundland, and other authorsreport similar observations with different sea urchinspecies (e.g. Jones and Kain 1967).

Higher densities of sea urchins outside the kelp bedsmight be explained in two ways. First, S. droebachienstsmight show poor growth and survival in areas coveredwith kelp, and the bare patches would represent thepreferred habitat. Alternatively, sea urchins mightgtaze yery actively on Laminaria and other species in thekelp forests, so that local high densities of sea urchinscan eliminate nearly all the macroscopic algae from apatch.

The evidence appears to support the second ex-planation. Himmelman and Steele (1971) report thatS. droebachiensis eats Laminana readily in the laboratory,an observation confirmed by R. J. Miller and J. Field(personal communication). Further, Miller observedthe recession of a Laminana bed in St. Margaret'sBay by 3 m in one year when there was an abundanceof sea urchins at the edge of it (Miller and Mann unpu-blished data). We have observed sea urchins within theLaminaria forest gTazing on the blades and the stipes.

The second explanation is also consistent with studiescarried out on other species of sea urchin. Recoloniza-tion of areas from which large macrophytes were absenthas been shown to follow the removal of S. purpuratus,S. franciscanus (Paine and Vadas 7969), Paracentrotuslividus (Kitching and Ebling 1.961), and Echinus esculentus(Kain and Jones 1966; Jones and Kain 1967). Cyclicalinteractions between Strongylocentrotus spp. and thegiant kelp Macro cy s t is hav e been extensively documented(Leighton et al. 1966; North 1969; Pearse et al. l97O;Leighton 1971). Moreover, sewer outfalls appear tomodify the intoraction by providing nourishment thatsupports sea urchin populations in the absence of kelp,thus preventing kelp reestablishment (Pearse et al.1970; Clark, 1969).

In Newfoundland, S. droebachiensis appears to controlthe distribution of Laminaria and Alaria (HimmelmanMS 1969). Macrophytes there are most abundant whereexposure to surf limits grazing by sea urchins, and onlyencrusting forms are found where high densities of seaurchins exist.

The control of sea urchin populations -Theseexamples show that locally dense populations ofsea urchins commonly overgraze their food supply.In areas where this does not occur. there must besome limitation on sea urchin population densityother than food supply, and the most probablemechanism is predation. Paine and Vadas (1969)

JOURNAL FISHERIES RESEARCII BOARD OF CANADA, YOL. 29, NO. 5, 1972

observed the starfish Pycnopodia reducing seaurchin density in local patches and suggested thatthis accounted for a patchwork of areas withoutsea urchins. In California, the sea otter (Enhydralutris nereis) is a major predator of sea urchins.A local increase in otters was followed by a sharpdecline in sea urchins and expansion of kelp beds(Mclean 1962; North 1965). Thus, in California,sea otters may be indirectly responsible for themaintenance of kelp forests.

The role of lobsters -The prime candidate forthe role of key predator on sea urchin populationsin eastern Canada is the lobster. Lobsters in tanksin this laboratory readily eat sea urchins. Himmel-man and Steele (1971) showed that two captivelobsters consumed 131 sea urchins, weighing a totalof 342 g, in 7 months. At the peak period they wereconsuming 28 g per lobster per day. Rock crabswere also active predators but 2 crabs consumedonly * of the biomass consumed by the lobsters.Also, they were restricted to urchins of 38 mm orless, while lobsters attacked urchins up to 72 mmin diameter.

Miller et al. (1971) reported that in a study of foodhabits of lobsters near Prince Edward Island, seaurchin remains occurred h 28/6 of the stomachsexamined. We have observed that, in the vicinityof lobster retreats in St. Margaret's Bay, sea urchinsare less abundant than elsewhere, and broken seaurchin tests often occur.

Herring gulls prey on sea urchins in the intertidalzone, and several species of fish are predators in thesublittoral zone (Himmelman and Steele 1971),but the only predator that is subject to intensehuman exploitation, and hence might be expectedto impart imbalance to the community, is the lob-ster. Reduction of lobster densities by fishing maybe responsible for allowing sea urchin populationsto reach densities high enough to clear away thekelp forest.

Anecdotal histories of several such sequences ofevents have been described to us by fishermen inSt. Margaret's Bay. Intensive lobster fishing pro-ceeded for several years in the same spot, afterwhich sea urchins became abundant and kelpdisappeared. This coincided with a period whenlobster fishing was poor (probably because ofoverfishing), and the fishermen stopped fishing inthe area until it returned to the normal situation:kelp forest, modest sea urchin abundance, and goodlobster fishing. This normally took about 3 or 4years.

If the foregoing ideas are correet, reduction inlobster population density below a certain criticallevel results not only in a decrease in the fishableJ.

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stock, but also in a population explosion of seaurchins, followed by overgrazing of the seaweedsthat are the basic producers in the whole system.The large areas of relatively barren rock reportedfrom many parts of the eastern coastline may, inone sense. be considered as man-made deserts.

Lobsters may then be considered as 'keystone'

species, (Paine 1966, 1969), in that their predatoryactivities are a key factor in structuring the com-munity.

Cu.nr, M. E. 1969. Dissolved free amino acids insea water and their contribution to the nutrition ofsea urchins, p, 70-93. In Kelp habitat improvementproject, annual report 1968-1969. Calif. Inst. Tech-nol.

Hrlrl,rewaN, J. H, MS 1969. Some aspects of theecology of Strongylocentrotus droebachiensis in easternNewfoundland. M.Sc.Thesis. MemorialUniversity,St. John's, Nfld. 125 p.

Hruuer,ulN, J. H., aNo D. H. Srrpr,s. 19'71. Foodsand predators of the green sea urchin Strongylocen-trotus droebachiensis in Newfoundland waters. Mar.Biol. (Berlin) 9: 315-322.

JoNrs, N. S., lNo J. M. KarN. 1967. Subtidal algalcolonization following the removal of Echinus. Hel-golaender Wiss. Meeresunters. 15: 460-/66,

KarN, J. M., awo N. S. JoNrs. 1966. Aleal coloniza-tion after removal of Echinus, p. 139-140. In E. G.Young and J. L. Mclachlan [ed.] Proceedings ofthe fifth international seaweed symposium. Perga-mon Press, Oxford,

KttcntNc, J, A., aNo F. J. EnI.nrc. 1961. The ecologyof Lough Ine, XI. The control of algae by Para-centrotus lividus (Fchinoidea). J. Anim. Ecol. 30:373-383.

LnrcuroN, D. L. 1971. Gnzing activities of benthicinvertebrates in southern California kelp beds, p.

421-453. In W. J. North [ed.] The biology of giantkelp beds (Macrocystis)in California. Nova HedwigiaZ. Kryptogamenk. Suppl. 32.

LnrcsroN, D. L., L. G. JoNns, AND W. J. Nonur. 1966.Ecological relationships between giant kelp and seaurchins in southern California, p. 141-153. InE. G.Young and J. L. Mclachlan [ed.] Proceedings of thefifth international seaweed symposium. PergamonPress, Oxford.

MaNN, K. H. 1972. Ecological energetics of theseaweed zone in a marine bay on the Atlantic coast ofCanada. I. Zonation and biomass of seaweeds.Mar. Biol. (Berlin) 12: l-10.

McLsaN, J. H. 1962. Sublittoral ecology of kelp bedsof the open coast area near Carmel, California. Biol.Bull. (Woods Hole) 122:, 95-114.

MILLER, R. J., K, H, Mn'x, AND D. J. Scannarr.1971. Production potential of a seaweed-lobstercommunity in eastern Canada, J. Fish. Res, Bd,Canada 28:.1733-1738.

Nontr, W. J. t965. Urchin predation, p. 57-61. InKelp habitat improvement project, annual report1964-1965. Calif. Inst. Technol.

1969. Project activities in San Diego County,p. 6-28. In Kelp habitat improvement project, annualreport 1968-1969. Calif. Inst. Technol.

ParNp, R. T, 1966. Food web complexity and speciesdiversity. Amer. Natur. 100: 65-75.

1969. A note on trophic complexity and communitystability. Amer. Natur. 103: 9l-93.

ParNp, R. T., aNo R. L. Vaoas. 1969. The efectsof grazing by sea urchins, Strongylocentrotus spp., onbenthic algal populations. Limnol. Oceanogr. 14:710-719.

Pe.lnsr, J. S., M. E. Cr-anr, D. L. LrromoN, C. T,Mrrcnrr.r., AND W. J. Nonrn. 1910. Marinewaste disposal and sea urchin ecology. Appendix,p. 1-93. InKelp habitat improvement project, annualrcpofi 1969-1970. Calif. Inst. Technol.

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