Technical Note Oceanographic Determinants of the Abundance of Common Dolphins (Delphinus delphis) in the South of Portugal Joana Castro 1,2,* , Ana Couto 2,3, Francisco O. Borges 2 , André Cid 1, Marina I. Laborde 1,2, Heidi C. Pearson 4 and Rui Rosa 2 1 AIMM—Associação para a Investigação do Meio Marinho, 1500-399 Lisboa, Portugal; [email protected] (A.C.); [email protected] (M.I.L.) 2 MARE—Marine and Environmental Sciences Centre, Faculdade de Ciências da Universidade de Lisboa, 1749-016 Lisboa, Portugal; [email protected] (A.C.); [email protected] (F.O.B.); [email protected] (R.R.) 3 CIBIO/InBIO—Universidade do Porto, 4485-661 Porto, Portugal 4 University of Alaska Southeast, Juneau, AK 99801, USA; [email protected] * Correspondence: [email protected] Received: 5 July 2020; Accepted: 25 August 2020; Published: 28 August 2020 Abstract: Off mainland Portugal, the common dolphin (Delphinus delphis) is the most sighted cetacean, although information on this species is limited. The Atlantic coast of Southern Portugal is characterized by an intense wind-driven upwelling, creating ideal conditions for common dolphins. Using data collected aboard whale-watching boats (1929 sightings and 4548 h effort during 2010–2014), this study aims to understand the relationships between abundance rates (AR) of dolphins of different age classes (adults, juveniles, calves and newborns) and oceanographic [chlorophyll a (Chl-a) and sea surface temperature (SST)] variables. Over 70% of the groups contained immature animals. The AR of adults was negatively related with Chl-a, but not related to SST values. The AR of juveniles was positively related with SST. For calves and newborns, although the relationship between SST and AR is similar to that observed for juveniles, the effect could not be distinguished from zero. There was no relationship between Chl-a levels and the AR of juveniles, calves and newborns. These results corroborate previous findings that common dolphins tend to occur in highly productive areas demonstrating linkages between their abundance and oceanographic variables, and that this region may be a potential nursery ground. Keywords: ecology; oceanography; Portugal; abundance rate; nursery; common dolphin; Delphinus delphis 1. Introduction The abundance and distribution of cetaceans is influenced by a series of oceanic and environmental variables [1,2]. Several studies worldwide have demonstrated these relationships, and have shown that cetacean movements vary within and between species [2] and are influenced by numerous variables, including sea-surface temperature [3,4], salinity [3], depth [5], seabed gradient [6], thermocline [7], oxygen minimum layer [8] and prey availability [9]. As top predators, cetacean distribution is closely related to the distribution of their prey [10–12], which in turn can be affected by upwelling systems [13–15] and SST [1]. Thus, in order to understand the factors driving cetacean distribution, insight into these environmental variables is needed. The Iberian Peninsula constitutes an excellent scenario for conducting ecological niche studies of small cetaceans, because it constitutes a transition area between two distinct environments (i.e., the Mediterranean Sea and the North Atlantic Ocean), and thus exhibits a high level of habitat Oceans 2020, 1, 165–173; doi:10.3390/oceans1030012 www.mdpi.com/journal/oceans Oceans 2020, 1, FOR PEER REVIEW 2 The Iberian Peninsula constitutes an excellent scenario for conducting ecological niche studies Oceans 2020, 1 166 of small cetaceans, because it constitutes a transition area between two distinct environments (i.e., the Mediterranean Sea and the North Atlantic Ocean), and thus exhibits a high level of habitat complexitycomplexity [16,17 [16,17].]. The coastalThe coastal region region off southwest off southwest Iberia Iberia is part is ofpart the of North the North Atlantic Atlantic eastern eastern boundary, boundary, which encompasses the northern branch of the Canary/Iberian Eastern Boundary which encompasses the northern branch of the Canary/Iberian Eastern Boundary Upwelling System Upwelling System (EBUS)—one of the world’s foremost productive marine ecosystems [18] and the (EBUS)—one of the world’s foremost productive marine ecosystems [18] and the western part of the western part of the northern margin of the Gulf of Cadiz [19,20] (Cape St. Vicente point—Sagres, see northernFigure margin 1). This of region the Gulf is characterized of Cadiz [19, 20by] an (Cape intense St. wind-driven Vicente point—Sagres, upwelling season see Figure spanning1). This from region is characterizedMarch to October, by an intense and by wind-driven an upwelling upwelling center in the season area spanningof Cape St. from Vicente March [21]. to These October, features and by an upwellingrestrict center sardines in the (Sardina area of pilchardus Cape St.), Vicente one of [the21]. main These prey features species restrict of small sardines cetaceans (Sardina (e.g., pilchardus common), one of thedolphins) main prey off species Portugal of [ small22,23], cetaceans to coastal (e.g.,waters. common dolphins) off Portugal [22,23], to coastal waters. FigureFigure 1. Map 1. Map of the of the study study area area in in southern southern PortugalPortugal with bathymetric bathymetric lines lines in meters. in meters. Each Each black black dot dot representsrepresents one one common common dolphin dolphin (Delphinus (Delphinus delphis delphis)) sighting.sighting. The southThe south coast coast of theof the Iberian Iberian Peninsula Peninsula has has been been thethe target of of many many studies studies that that have have revealed revealed a rich cetaceana rich cetacean biodiversity biodiversity [24– 26[24–26].]. However, However, knowledge knowledge concerning concerning thethe distribution and and abundance abundance of cetaceansof cetaceans off the off Atlantic the Atlantic coast coast of the of the Iberian Iberian Peninsula Peninsula remainsremains scarce scarce and and often often limited limited to specific to specific areas [17,24,25,27]. In particular, few studies have examined cetacean occurrence in the waters off areas [17,24,25,27]. In particular, few studies have examined cetacean occurrence in the waters off southern Portugal [17]. The common dolphin (Delphinus delphis) is the most abundant cetacean southern Portugal [17]. The common dolphin (Delphinus delphis) is the most abundant cetacean species in this area [17], although very little is known regarding their biology and ecology, including speciestheir in abundance this area [ 17and], althoughdistribution. very Further, little is there known is limited regarding information their biology regarding and the ecology, effect of including the theiraforementioned abundance and environmental distribution. variable Further,s on there common is limited dolphin information distribution [ regarding17]. Common the dolphins effect of the aforementionedoccur in most environmental coasts of the world, variables and mainly on common in the continental dolphin distributionshelf but can [be17 found]. Common in all depth dolphins occurranges in most [28–31]. coasts The of common the world, dolphin and mainlyis described in the as a continental highly gregarious shelf butspecies, can forming be found groups in all of depth rangesup [ 28to –several31]. The hundreds common or even dolphin thousands is described of individuals, as a highly although gregarious presenting species, a basic forming social unit groups of of up tobetween several 20 hundreds to 30 individuals or even per thousands group [32]. of individuals,Generally regarded although as an presenting opportunistic a basic feeder social [10,33], unit of betweenthis species 20 to 30 is individualsconsidered a perpreferential group [ 32ichtyophageous]. Generally regardedin the South as of an Portugal opportunistic [34], in the feeder Bay [of10 ,33], Biscay [35,36] and in the Mediterranean [37]. this species is considered a preferential ichtyophageous in the South of Portugal [34], in the Bay of The apparent lack of studies on cetaceans off southern Portugal could be related to the logistical Biscay [35,36] and in the Mediterranean [37]. constraints of performing dedicated cetacean surveys. Such surveys are demanding in terms of the Thefinancial apparent resources, lack time, of studies and personnel on cetaceans required, off southernand thus researchers Portugal couldoften utilize be related alternative to the study logistical constraintsapproaches, of performing such as the dedicated use of platforms cetacean of surveys. opportunity Such (e.g., surveys whale-watching are demanding vessels in terms[31,38]). of the financialAlthough resources, there are time, inherent and personnel limitations required, to this approach and thus (e.g., researchers the potential often interaction utilize alternative between the study approaches,vessel and such cetaceans, as the use and of platformsthe influence of opportunity of sea state (e.g.,and whale-watchingwind direction on vessels the sighting [31,38]). ability, Although theredistribution are inherent and limitations behaviour to of this the approach animals (e.g.,[38]), theplatforms potential of interactionopportunity betweencan yield the valuable vessel and cetaceans,information and the regarding influence cetacean of sea ecology state and that windwould direction otherwiseon be theunobtainable. sighting ability, distribution and behaviour of the animals [38]), platforms of opportunity can yield valuable