Seasonal presence of tidepool fish species in a rocky intertidal zone of northernCalifornia, USA

J . R. Moring 1Department of Fisheries, Humboldt State University, Arcata, California 95521, USA

Keywords: fishes, intertidal, seasonality, California

Abstract

An intertidal fish community in Trinidad Bay, Humboldt County, California, was studied between Novem-ber 1968 and May 1970 . The nursery function of rocky tidal pools for juveniles of subtidal fishes was indicat-ed by peaks in indices of seasonally abundant fish species in summer . The proportion of juveniles of seasonalspecies in the intertidal fish community rose significantly in collections of late spring and summer, to a maxi-mum of 35% of the individuals collected in July. Annual dominance was relatively high due to abundantpopulations of the tidepool sculpin, Oligocottus maculosus, but two other fish species were also encounteredin tidepools every month of the year .

Introduction

The rocky intertidal zones of the world containfishes that are year-round residents, fishes that uti-lize tidepools as transient visitors, and fishes thatare present seasonally in tidepools, generally asjuveniles (an excellent review is presented by Gib-son, 1982) . These fishes, like all members of the in-tertidal community, are influenced by both physicaland biological parameters . Though Sanders (1969)suggested temperate, rocky intertidal regions are af-fected more by physical conditions, the evidence isnot clear. Menge (1976), in a review of the literature,concluded that biological factors were often themajor influence on such intertidal communities .

Although seasonally influenced by physicochem-ical characteristics of the environment, speciesdiversities of intertidal fish communities in tropicaland subtropical waters may remain relatively simi-lar from year to year (Thomson & Lehner, 1976 ;

I Present address : Maine Cooperative Fish and Wildlife Re-search Unit, Department of Zoology, University of Maine, Oro-no, Maine 04469, USA

Hydrobiologia 134, 21-27 (1986) .© Dr W. Junk Publishers, Dordrecht . Printed in the Netherlands .

Van Tassell & Dooley, 1979 ; Allen & Horn, 1980) .In cooler waters, the seasonal appearance oftidepool fish species can be more pronounced . Sig-nificant annual variations in species richness oftidepool fishes can also occur in localized areas asa consequence of human activities, such as over-collection, pollution, and the indirect effects ofshoreline structures (Ghazanshahi et al., 1983 ;Moring, 1983). The reduced diversity of intertidalinvertebrates and marine algae can result in reduceddiversity of intertidal fishes .

Along the California coast, a thermal transitionzone between Point Conception and Cape Men-docino results in a biological transition from tropi-cal/subtropical - temperate tidepool fish speciesto strictly temperate - boreal forms (Greeley, 1899 ;Hubbs, 1948; Morris, 1960, 1961 ; Horn & Allen,1978; Seapy & Littler, 1980 ; Yoshiyama, 1981) .Trinidad Bay, California, is located north of thiszone, and rocky intertidal fish communities mightbe expected to exhibit seasonality of species com-position . This seasonality would be further ampli-fied if Trinidad Bay tidepools serve as nurserygrounds for juveniles of offshore fishes, as has

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been noted for rocky tidepool areas of southernCalifornia (Williams, 1957 ; Norris, 1963), the Gulfof California (Thomson & Lehner, 1976), Portugal(Arruda, 1979), and the coast of Maine (Moring,1985), and for tidepools within estuarine - oysterreef areas of South Carolina (Crabtree & Dean,1982) .

Chadwick (1976) derived diversity and species in-dices for intertidal fishes at a site near Cape Men-docino, California, and at a site on Vancouver Is-land, British Columbia, but the estimates werecomputed only from collections made in late July .A community influenced by wide seasonality in en-vironmental conditions might be expected to showseasonal fluctuations in species richness and domi-nance. A study was conducted in Trinidad Bay,California, to examine the seasonal appearance oftidepool fish species and define the importance ofrocky tidepools as nursery areas for juveniles ofoffshore fishes .

Materials and methods

Trinidad Bay, California, is located in HumboldtCounty, 23 km north of Humboldt Bay (lat 41 °31'N; long 124' 8'W). It is semi-protected, andcharacterized by rocky shores and scatteredtidepools. Fishes were collected within the rockpools of seven distinct and scattered intertidal areaswithin the bay. Tidepool fishes were collected dur-ing 53 sampling trips between November 1968 andMay 1970. Trips were made exclusively at low tide(-0.7 to +0.3 m height), and the duration of col-lecting periods averaged 80 minutes per area . Onlyone area was sampled on a single date.

Fishes were captured by hand nets in tidepoolsthroughout each area. An attempt was made tocapture all fishes encountered throughout eacharea, though a complete census could not be ob-tained with such methods . No poisons, collectinganesthetics, or pumps were used, yet each pool andits cover within each area was sampled on eachdate. Most California tidepool fishes exhibit differ-ent degrees of habitat partioning (Williams, 1954 ;Graham, 1970; Nakamura, 1976a, 1976b ; Burgess,1978), and each type of habitat was sampled withineach area. The resultant collection, though stronglyrepresentative of the species present, was not ran-dom or complete ; it was, rather, a collection of

fishes encountered (size 10 mm and above) in amethodical examination of habitats within eacharea, and is an indication of seasonal changes inappearance of these fishes. Some rare or uncom-mon species, or species adapted to highly specificsubhabitats could have been missed .

Though no single diversity index can adequatelydescribe fish populations in a seasonally-fluctuating environment, such indices are useful forcomparing population structures in differentlocales, and three such indices were computed . TheShannon-Wiener Index (Shannon & Weaver, 1962)is sensitive to the distribution of individuals by spe-cies, while the Species Richness Index (Margalef,1968) gives weight to species number . An effectivestatistical comparison of populations dominatedby a single species much of the year is the Domi-nance Measure of Simpson (1949), though thismeasurement can also be influenced by the suscep-tibility of a species to the collecting equipment ortechniques .

Results

Twenty species of tidepool fishes have been listedfor Trinidad Bay, though the presence of at least sixother species is considered probable (Moring,1972) . Of these, eight (40%) were encountered dur-ing most months of the year. Of the 1599 tidepoolfishes collected, the three most common species(Oligocottus maculosus, O, snyderi, Gobiesoxmaeandricus) were found every month of the year(Table 1) . Oligocottus maculosus accounted for69% of the fishes collected intertidally in TrinidadBay. Simpson's Measure of Dominance (C'), an in-dication of the dominant presence of a specieswithin a community, indicates C'=0 .495 andD=0.505 (ranges: C'=0.325 to 0.679; D=0.321 to0.675), a relatively high dominance of this principalspecies (C'=1 .0 would indicate a single speciescommunity, one of lowest diversity) . The propor-tion of juveniles of subtidal species in this intertidalfish community rose significantly in late spring andsummer (Table 2), thus indicating an importantnursery function, particularly in May, June, andJuly. These juveniles were not strictly transient in-dividuals. Marked Sebastes melanops were recap-tured at least 37 days later in the same intertidalarea (Moring, 1976, and unpubl . data) .

Table 1 . Monthly occurrence of rocky intertidal fish species in Trinidad Bay, California .

a Found intertidally n other areas of Humboldt County or southern Oregon in these months (Moring, unpubl . data) .

Tidepool fishes of the Bay can be subjectivelygrouped into six categories (Table 3) . Those fishesclassified as seasonal were found intertidally onlyas juveniles, and their presence was primarily be-tween May and September (six species: 30%) . Scor-paenichthys marmoratus was found intertidallyduring most of the year, but only in juvenile stages ;adults in tidepools were uncommon .

Table 2. Proportion of juveniles of all seasonal species oftidepool fishes in the Trinidad Bay community, by month .

Month

Percentage of seasonal juveniles

January 0February 0March 0April 2May 13June 18July 35August 3September 0October 1November 1December

2

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Numbers of species changed during the year(Fig. 1), reflecting the immigration of juveniles ofsubtidal species in spring, and their emigration inSeptember. The measurements of species richness(D'), that use pooled monthly data and give weightto numbers of species, reflect these same seasonalfluctuations . Species richness was significantly low-er in fall months (September-November) than inother seasons, at the 95 010 confidence level(X2 =8.10), and this reflects the emigration of sea-sonal species . Species diversity indices (Shannon-Wiener : H'), more of a reflection of the communi-ty structure, averaged 1 .21 for the yearly pooleddata; peaking in winter (1 .32-1 .49) and early sum-mer (1 .31-1 .38) and declining in early spring(0.90-1.12) and early fall (0 .65-0.95) .

Discussion

The most important species from Trinidad Baytidepools, Oligocottus maculosus, composed 69%of those fishes collected . This value is to some ex-tent a reflection of sampling methods and the visu-

Species Month

D

N

J F M A M J J A S 0 N

Oligocottus maculosus X X X X X X X X X X X X 1102Oligocottus snyderi X X X X X X X X X X X X 195Gobiesox maeandricus X X X X X X X X X X X X 77Sebastes melanops X X X X 77Xiphister atropurpureus X X X X X X X X X X 38Scorpaenichthys marmoratus X X X X X X X X 25Anoplarchus purpurescens X X X X X X X X X X 21Xererpes fucorum X X X X X X X X 15Apodichthys flavidus X X X X X X 11Hexagrammos decagrammus X X 8Citharichthys stigmaeus X X X 6Clinocottus globiceps X X X 6Clinocottus acuticeps X 5Sebastes mystinus X X X X 4Artedius lateralis a X 2Liparis florae X a a X 2Hemilepidotus spinosus X 2Artedius fenestralis X 1Pholis ornata X 1Cebidichthys violaceus X 1

Number of collecting trips 3 6 6 6 7 3 4 4 2 3 4 5 53

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Table 3 . Groupings of fishes by seasonal residence and abun-dance in tidepools of Trinidad Bay, California .

Group I

Common, year-round residents of intertidalzone :Anoplarchus purpurescensGobiesox maeandricusOligocottus maculosusOligocottus snyderiXiphister atropurpureus

Group II

Moderately common, year-round residents of in-tertidal zone :Apodichthys flavidusXererpes fucorum

Group III Uncommon, year-round residents of intertidalzones :Cebidichthys violaceusClinocottus globicepsLiparis florae

Group IV

Juveniles seasonally present in the intertidalzoneb :Citharichthys stigmaeusHemilepidotus spinosusHexagrammos decagrammusSebastes melanopsSebastes mystinus

Group V

Juveniles present year-round in the intertidalzone :Scorpaenichthys marmoratus

Group VI

Inshore marine fishes found rarely in the inter-tidal zone:Artedius fenestralisArtedius lateralisClinocottus acuticepsPholis ornata

a May also include Ascelichthys rhodorus, Leptocottus arma-tus, and Oligocottus rimensis (Moring, 1972), as well as severalother species .b May also include Embiotoca lateralis, Parophrys vetulus, Se-bastes paucispinus (Moring, 1972) ; Hemilepidotus hemilepido-tus, Hexagrammos superciliosus, Sebastes pinniger, andXiphister mucosus (Chadwick, 1976), and other species of Se-bastes .

al observation of this species (this cottid is widelydispersed in many types of habitats and tends touse protective cover less than other species) . But,this proportion is similar to that obtained by Green(1971) and Chadwick (1976) near Port Renfrew,British Columbia (59 and 60%, respectively), butconsiderably higher than the proportion reportedby Chadwick (41%) at Bruels Point, California(215 km to the south of Trinidad Bay) . Each ofthese collections (Green, 1971 ; Chadwick, 1976)was made in July only, a time when many seasonalspecies are present, thus reducing the proportion of

MONTH

Fig. 1. Seasonal fluctuations in numbers of species (S) and Spe-cies Richness (D') for tidepool fishes of Trinidad Bay, Califor-nia ; pooled for the years 1968-1970 .

resident Cottidae . In July collections in TrinidadBay, O. maculosus accounted for 60% of the fishescollected, about the same proportion as that col-lected by others in more northern waters .

Bruels Point is also located on the edge of thePoint Conception - Cape Mendocino thermaltransition zone, and intertidal fish communitiesthere would include more sub-tropical/temperatespecies, particularly in July . A significant propor-tion (15%) of the Bruels Point collection of Chad-wick (1976) was the canary rockfish, Sebastes pin-niger, a temperate species not collected in TrinidadBay. Without this seasonal species, the Bruels Pointfish community in July would have been composedof 48% Oligocottus maculosus. Although adult Se-bastes pinniger are relatively common off thenorthern California coast, and north to BritishColumbia (Miller & Lea, 1972), they have not beenreported from Humboldt Bay, located nearTrinidad Bay (Gotshall et al., 1980). Whether byhabitat preference, or thermal requirements,juveniles of this offshore species were seasonallyimportant to the intertidal zone of Bruels Point,but not to the rocky intertidal zone of TrinidadBay.

Chadwick (1976) used the slightly differentWilhm-Dorris Diversity Index (Wilhm & Dorris,1968) for computing diversity of the intertidal fishcommunity at Bruels Point and Port Renfrew .Computing a similar index (d') from Trinidad Baydata, overall fish diversity was higher in Trinidad

2.62 .21 .8 W1 .4 z

V1 .0 m0 .6

Bay tidepools (d' = 1 .79) than those at Bruels Point(1 .22) or Port Renfrew (1 .30) . However, Chadwick'scollections were made only in July, and the July d'at Trinidad Bay was lower (1 .47) .

The seasonal peaks in species richness forTrinidad Bay pools are similar to those exhibited byan estuarine fish community in southern Californiastudied by Allen & Horn (1975) . However, the twotypes of environments are different, and peaks inabundance, diversity, and richness in the estuary dooccur in winter (Allen & Horn, 1980 ; L.G. Allen,Department of Biology, California State Universi-ty, Northridge, pers . commun.). Sampling successmay be a factor with some species in Trinidad Bay,but changes in abundance reflect increases in themost common species, Oligocottus maculosus(Moring, 1976), due to recruitment of juveniles .However, regression analyses suggest that suchabundance was not the significant factor affectingdiversity.Because no collections were made during ebb

tides at night, the possibility of a diurnal variationin diversity and richness of fish species inhabitingrocky tidal pools is not excluded . Several species ofsubtidal fishes move into the rocky intertidal orbsandy intertidal during flood tides, and move off-shore with the receding tide (Bolin, 1964 ; Tyler,1971; Gibson, 1972, 1982; Gibson & Hesthagen,1981; Middaugh & Takita, 1983), and are intertidal,rather than tidepool fishes . However, most analysesof fish communities in tidepools were also made atebb tide, hence collections of Trinidad Bay fishesare valid for comparison with other rocky intertidalareas and for the qualitative identification of inter-tidal species .

Most estimates of diversity or population size re-quire random or complete sampling . This was notthe case for this study. If sampling was restricted toonly portions of an intertidal zone, the data wouldbe skewed toward species dominating thosehabitats . And, estimates of diversity or populationsize derived from collections in only a few restric-tive habitats within the intertidal zone could not beaccurately compared with estimates derived frommeasurements made elsewhere . I believe thatreasonably accurate data were obtained by the col-lection procedures employed, and by samplingthroughout an intertidal area . The similar propor-tions of cottids found in the July intertidal fishcommunities of Port Renfrew, British Columbia

25

(Green, 1971 ; Chadwick, 1976) and that of TrinidadBay, and the similarity of diversity indices betweenTrinidad Bay and Bruels Point, California (Chad-wick, 1976) indicates the sampling did not overlyskew collection data toward the more easily ob-served Oligocottus maculosus. The collection indozens of pools in seven different areas of TrinidadBay provided habitat diversity, though the diversityand density of fishes collected may only approxi-mate the complete community structure in theTrinidad Bay intertidal .

The seasonal appearance of juveniles of mem-bers of the Bothidae, Cottidae, Embiotocidae,Pleuronectidae, and Scorpaenidae in Trinidad Baytidepools reinforces observations made in manyother rocky intertidal regions of the world of therole of tidepools as seasonal nursery areas .Tidepools in rocky coastal areas have importantseasonal links with the early life history of manyfishes .

Summary

Though species diversities of tidepool fishes intropical and subtropical waters often remain stablefrom year to year, seasonality can be pronounced inpopulations in cooler waters. From collectionsmade in dozens of tidepools in seven rocky intertid-al areas of Trinidad Bay, northern California, USA,significant seasonal trends in species diversity and,particularly, species richness were noted .

Though the collections were not complete, orstrictly randomized, sampling methods weremethodical. Much of the seasonality in numbers ofspecies is manifested by the utilization of tidepoolsby juveniles of offshore fishes during the spring-to-autumn period. Species richness increases duringthis time, and decreases in winter when only resi-dent species are present .

Twenty species of fishes were collected fromtidepools, though the presence of several other spe-cies is likely. The tidepool sculpin, Oligocottusmaculosus, dominated the collections (C'=0 .495),and, along with 0 snyderi and Gobiesox maeandri-cus, was found in tidepools every month of theyear. Population densities and diversities of fishesindicate rocky intertidal pools have seasonal im-portance .as nursery areas for juveniles of offshorespecies .

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Acknowledgments

Thanks are due G. H . Allen, Humboldt StateUniversity, for advice on aspects of this research ;and to D. A. Misitano, National Marine FisheriesService, Mukilteo, Washington ; W. E . Glanz,Department of Zoology, University of Maine ; andJ. G. Stanley, US Fish and Wildlife Service, AnnArbor, Michigan, for critically reviewing an earlierversion of the manuscript .

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