Ocean Sci., 14, 1461–1482, 2018 https://doi.org/10.5194/os-14-1461-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Linking sardine recruitment in coastal areas to ocean currents using surface drifters and HF radar: a case study in the Gulf of Manfredonia, Adriatic Sea Roberta Sciascia1, Maristella Berta1, Daniel F. Carlson1,6,7, Annalisa Griffa1, Monica Panfili8, Mario La Mesa8, Lorenzo Corgnati1, Carlo Mantovani1, Elisa Domenella8, Erick Fredj3, Marcello G. Magaldi1,2, Raffaele D’Adamo9, Gianfranco Pazienza9, Enrico Zambianchi1,4, and Pierre-Marie Poulain5 1Istituto di Scienze Marine (ISMAR), Consiglio Nazionale delle Ricerche (CNR), La Spezia, Italy 2Johns Hopkins University, Department of Earth and Planetary Science, Baltimore, MD, USA 3Department of Computer Sciences, Jerusalem College of Technology, Jerusalem, Israel 4DiST, Università degli Studi di Napoli ”Parthenope” and CoNISMa, Naples, Italy 5Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste, Italy 6Department of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee, FL, USA 7Arctic Research Centre, Department of Bioscience, Aarhus University, Aarhus, Denmark 8Istituto per le Risorse Biologiche e le Biotecnologie Marine (IRBIM), Consiglio Nazionale delle Ricerche (CNR), Ancona, Italy 9Istituto per le Risorse Biologiche e le Biotecnologie Marine (IRBIM), Consiglio Nazionale delle Ricerche (CNR), Lesina, Italy Correspondence: Roberta Sciascia ([email protected]) Received: 21 May 2018 – Discussion started: 6 June 2018 Revised: 24 October 2018 – Accepted: 15 November 2018 – Published: 27 November 2018 Abstract. Understanding the role of ocean currents in the work. Transport to the Gulf of Manfredonia from remote SAs recruitment of commercially and ecologically important fish seems more likely than local spawning and retention given a is an important step toward developing sustainable resource mismatch between observed PPLDs of 30–50 days and rel- management guidelines. To this end, we attempt to elucidate atively short ( < 10-day) average residence times. The num- the role of surface ocean transport in supplying recruits of ber and strength of connections between the gulf and remote European sardine (Sardina pilchardus) to the Gulf of Man- SAs exhibit a strong dependence on PPLD. For PPLDs of fredonia, a known recruitment area in the Adriatic Sea. Sar- 20 days or less, the gulf is connected to SAs on the west- dine early life history stages (ELHSs) were collected dur- ern Adriatic coast through transport in the Western Adriatic ing two cruises to provide observational estimates of their Current (WAC). SAs on the east coast are more important at age–size relationship and their passive pelagic larval dura- longer PPLDs. SAs in the northern and central Adriatic ex- tion (PPLD). We combine these PPLDs with observations hibit weak connections at all PPLD ranges considered. These of surface ocean currents to test two hypotheses: (1) ELHSs results agree with otolith microstructure analysis, suggesting are transported from remote spawning areas (SAs) by ocean that the arrival of larvae in the gulf is characterized by re- currents to the Gulf of Manfredonia; (2) sardines spawn lo- peated pulses from remote SAs. This is the first attempt to de- cally and ELHSs are retained by eddies. A historical surface scribe the processes related to Lagrangian connection to, and drifter database is used to test hypothesis 1. Hypothesis 2 is retention in, the Gulf of Manfredonia that will be comple- tested by estimating residence times in the Gulf of Manfre- mented in the future using validated numerical ocean models donia using surface drifters and virtual particles trajectories and biophysical models. that were computed from high-resolution observations of sur- face currents measured by a high-frequency (HF) radar net- Published by Copernicus Publications on behalf of the European Geosciences Union. 1462 R. Sciascia et al.: Sardine recruitment in coastal areas 1 Introduction cific ecosystem knowledge, but despite the economic and ecological importance of the gulf, many processes are still Globally, pelagic forage fish, like sardines, provide important only partially understood (Specchiulli et al., 2016). For ex- ecosystem services through their transfer of energy between ample, the mechanisms of sardine recruitment in the gulf trophic levels (Essington et al., 2015) and by supporting a are not clear yet; i.e., there is no factual evidence that sug- commercial industry valued at approximately USD 17 billion gests that the nursery is supported by local spawning or by (Pikitch et al., 2014). European sardine (Sardina pilchardus) remote spawning areas. This is a relevant question, since dif- represents one of the most important pelagic fish resources in ferent scenarios could call for different management strate- the Mediterranean Sea. Sardines and anchovies are the pri- gies (Fogarty and Botsford, 2007). mary target species in purse-seine and mid-water pair trawl In this paper, we contribute to the understanding of the gulf fisheries in the Adriatic Sea, with annual catches that fluc- recruitment processes by investigating the role of ocean cur- tuated between ∼ 21000 t in 2005 and ∼ 79000 t in 2016 rent transport pathways. We focus on eggs and initial larval (SAC-GFCM, 2016). Intense fishing pressure on sardines in stages during which ELHSs can be assumed to be passively the Adriatic Sea resulted in large fluctuations in catches over advected by the currents, and we estimate the corresponding the last 40 years and culminated in a collapse of the fish- passive pelagic larval duration (PPLD; Shanks, 2009) based ery in the late 1980s (Morello and Arneri, 2009; Lotze et al., on the analysis of sardine ELHS specimens collected in the 2011). Catches have since increased but the stock remains Gulf of Manfredonia. Passive transport is then computed overexploited, with biomass values above the precautionary for different PPLDs using ocean current data, with the goal reference point (SAC-GFCM, 2016; Lotze et al., 2011). Ad- of testing the following two hypotheses: (1) ELHSs are re- ditionally, sardines constitute an important shared fishery re- motely spawned and transported from remote SAs by ocean source between countries along the Adriatic coast (Albania, currents to the Gulf of Manfredonia; (2) sardines spawn lo- Bosnia, Croatia, Italy, Montenegro, and Slovenia), making cally and ELHSs are retained by eddies and other small-scale marine spatial planning (e.g., Carpi et al., 2017; Depellegrin retentive circulation features. The historical surface drifter et al., 2017) particularly difficult. database in the Adriatic Sea (Poulain et al., 2013) that pro- In addition to fishing pressure, environmental conditions vides direct measurements of transport pathways is used to (e.g., temperature, salinity, and ocean currents) also influence test hypothesis 1. Hypothesis 2 is tested by estimating res- the stock variability of short-lived pelagic species by im- idence times of drifters and of virtual particles trajectories pacting the survival of early life history stages (ELHSs) and computed from high-frequency (HF) radar observations of therefore recruitment strength (Peterson and Wroblewski, surface currents in the Gulf of Manfredonia (Corgnati et al., 1984; Bradford, 1992; Bakun, 1996; Regner, 1996; Coombs 2018) during the sardine spawning and recruitment season et al., 2003; Santojanni et al., 2006; Garrido et al., 2017). (September–May). In general, oceanographic processes, and their modulation This is the first study that addresses the impact of ocean by high- and low-frequency variability, are fundamental in currents on recruitment in the gulf using information based driving dispersal and retention of ELHSs and can affect the on actual current measurement data. Recent studies made in- spawning habitat and behavior of adults, as well as the sur- ferences about circulation in the gulf from relatively coarse- vival of eggs and larvae, largely contributing to recruitment resolution (1=16◦) ocean models (Bray et al., 2017; Specchi- variability (Lasker, 1981; Boehlert and Mundy, 1994; Gov- ulli et al., 2016) but did not directly measure currents. High- oni and Pietrafesa, 1994; Sabatés and Olivar, 1996; Hare resolution numerical simulations in the vicinity of the gulf et al., 2002; Sanchez-Velasco et al., 2002; Santos et al., 2004, have largely focused on instabilities of the buoyant bound- 2018). In particular, ocean currents and their spatiotemporal ary current and not specifically on the dynamics within the variability can impact sardine recruitment during the disper- gulf (Burrage et al., 2009; Magaldi et al., 2010), while a sal stage, when eggs and developing larvae can be treated, first preliminary study on large-scale dispersal of eggs and at least to some degree, as passive (Largier, 2003). Thus, larvae has been performed using velocity fields from a high- identifying transport pathways from spawning areas (SAs) to resolution Regional Ocean Modeling System (ROMS) model recruitment areas is a necessary step towards understanding of the Adriatic Sea (Gramolini et al., 2010). complex physical–biological interactions. The rest of the paper is organized as follows. Section 2 The Gulf of Manfredonia is a well-known sardine nurs- summarizes the surface circulation of the Adriatic Sea, phys- ery area that is located in the southwestern Adriatic Sea ical setting of the Gulf of Manfredonia, and the sardine pop-