Population dynamics of the Argentinean surf clams Donax hanleyanus and Mesodesma mactroides from open-Atlantic beaches off Argentina Populationsdynamik der Argentinischen Brandungsmuscheln Donax hanleyanus und Mesodesma mactroides offener Atlantikstrände vor Argentinien Marko Herrmann Dedicated to my family Marko Herrmann Alfred Wegener Institute for Polar and Marine Research (AWI) Section of Marine Animal Ecology P.O. Box 120161 D-27515 Bremerhaven (Germany) [email protected] Submitted for the degree Dr. rer. nat. of the Faculty 2 of Biology and Chemistry University of Bremen (Germany), October 2008 Reviewer and principal supervisor: Prof. Dr. Wolf E. Arntz 1 Reviewer and co-supervisor: Dr. Jürgen Laudien 1 External Reviewer: Dr. Pablo E. Penchaszadeh 2 1 Alfred Wegener Institute for Polar and Marine Research (AWI) Section of Marine Animal Ecology P.O. Box 120161 D-27515 Bremerhaven (Germany) 2 Director of the Ecology Section at the Museo Argentino de Ciencias Naturales (MACN) - Bernardino Rivadavia Av. Angel Gallardo 470, 3° piso lab. 80 C1405DJR Buenos Aires (Argentina) Contents 1 Summary .................................................................................................... 5 1.1 English Version ........................................................................................... 5 1.2 Deutsche Version ........................................................................................ 9 1.3 Versión Español ........................................................................................ 13 2 Introduction ............................................................................................. 17 2.1 Sandy Beach Ecosystems ......................................................................... 17 2.2 The Wedge Clam ...................................................................................... 18 2.3 The Yellow Clam ....................................................................................... 23 2.4 Aims .......................................................................................................... 27 3 Materials and Methods ............................................................................ 28 3.1 Study Sites ................................................................................................ 28 3.2 Population Structure .................................................................................. 33 3.3 Growth Estimation ..................................................................................... 35 3.4 Reproduction ............................................................................................. 38 4 Summary of Results ................................................................................ 41 4.1 Population Structure .................................................................................. 41 4.2 Growth Estimation ..................................................................................... 48 4.3 Reproduction ............................................................................................. 53 5 General Discussion and Conclusions ................................................... 58 5.1 Population Structure .................................................................................. 58 5.2 Growth Estimation ..................................................................................... 60 5.3 Reproduction ............................................................................................. 71 6 Publications ............................................................................................. 77 6.1 Publication I ............................................................................................... 79 6.2 Publication II ............................................................................................ 109 6.3 Publication III ........................................................................................... 131 6.4 Publication IV .......................................................................................... 159 6.5 Publication V ........................................................................................... 179 7 Appendix ................................................................................................ 198 7.1 Conference Presentations ....................................................................... 198 7.2 Supervision of Diploma Theses ............................................................... 199 7.3 Acknowledgments ................................................................................... 200 7.4 List of Selected Abbreviations ................................................................. 203 7.5 References .............................................................................................. 206 SUMMARY – ENGLISH VERSION 5 1 Summary 1.1 English Version The surf clams Mesodesma mactroides Reeve, 1854 and Donax hanleyanus Philippi, 1847 are the two dominating species in macrobenthic communities of sandy beaches off northern Argentina, with the latter now surpassing M. mactroides populations in abundance and biomass. Before stock decimation caused by exploitation (during the 1940s and 1950s) and mass mortality events (1995, 1999 and 2007) M. mactroides was the prominent primary consumer in the intertidal ecosystem and an important economic resource in Argentina. Since D. hanleyanus was not commercially fished and not affected by mass mortality events, it took over as the dominant species, but did never reach the former abundance of M. mactroides. Currently abundance and biomass of both surf clams are a multiple smaller than those of forty years ago, indicating the conservation status of D. hanleyanus and M. mactroides as endangered. Therefore the aim of this study is to analyse the population dynamics (population structure, growth and reproductive biology) of D. hanleyanus and M. mactroides, and to compare the results with historical data in order to detect possible differences within surf clam populations forty years ago and at present. Monthly quantitative sampling was carried out between December 2004 and December 2006 in the intertidal zone of Santa Teresita, Mar de las Pampas and Faro Querandí. All three sandy beaches display different exposures with contrasting morphodynamics and can be classified as sheltered-dissipative, exposed-intermediate and exposed-reflective, respectively. While D. hanleyanus inhabits all three beaches, M. mactroides was almost exclusively present at Santa Teresita. Whereas the latter is an endemic species to Argentina, living specimens of D. hanleyanus were first reported on Argentinean beaches in 1960, although fossils have been found from the Querandinense period in the early Holocene. Live individuals of D. hanleyanus measured between 3 and 40 mm and those of M. mactroides showed an anterior-posterior shell length (apSL) between 2 and 64 mm. Both surf clams were found associated with very few species and low abundance of other macrozoobenthic fauna. The gastropod Olivancillaria vesica auricularia was recorded as a 6 SUMMARY – ENGLISH VERSION potential predator of both surf clam species especially in Santa Teresita. At Faro Querandí the American oystercatcher Haematopus ostralegus was observed feeding on D. hanleyanus. Mean abundance of M. mactroides was recorded as 110 ind. m-2 and those of D. hanleyanus as 50 to 267 ind. m-2. The abundance of the latter was linked to sand grain size. Highest abundance was recorded at the reflective beach Faro Querandí, which is characterised by coarse sediment. The decrease of D. hanleyanus in summer periods appears to be principally related to human activities. The intertidal biomass of the surf clam populations ranged between 0.04 and 1.32 g shell-free ash-free dry mass (AFDM) m-2yr-1 (D. hanleyanus) and 0.06 to 0.07 g AFDM m-2yr-1 (M. mactroides). The individual production of D. hanleyanus revealed the highest value at 30 mm apSL (0.16 g AFDM m-2 yr-1), with an individual production of M. mactroides of 0.35 g AFDM m-2yr-1 at 47 mm apSL. Annual production for D. hanleyanus ranged between 0.08 and 0.99 g AFDM m-2yr-1 and for M. mactroides between 0.12 and 0.19 g AFDM m-2 yr-1, resulting in an annual renewal rate (P/̼Ŭ) of 0.82-2.16 and 1.84-2.93, respectively. A review of the renewal rate of several Donax species revealed that the renewal rate increased with decreasing latitude from temperate to tropical regions. Growth estimations based on length-frequency distributions (LFDs) confirmed that both surf clams are fast growing species (D. hanleyanus: -1 -1 L. = 44 mm, K = 0.46-0.47 yr ; M. mactroides: L. = 71 mm, K = 0.47 yr ), and both have a potential for aquaculture. Seasonally oscillating growth with slowest growth rates in austral autumn (WP = 0.45 ~ mid-May) were revealed for D. hanleyanus (C = 0.8) and weak seasonality with slowest growth rates occurring in austral winter (WP = 0.7 ~ mid-August) for M. mactroides (C = 0.1). Compared with growth studies from the 1970s, a slower growth of D. hanleyanus was estimated, but a higher maximum length was recorded. M. mactroides now grow faster but maximum length is lower. Life span of D. hanleyanus was calculated as 4.96-5.18 yrs, and that of M. mactroides was estimated at 6.19 yrs. Overall growth performance (OGP) indices of D. hanleyanus (OGP = 4.54-4.60) and M. mactroides (OGP = 5.23-5.24) resulting from estimated growth parameters showed values comparable with SUMMARY – ENGLISH VERSION 7 those of other temperate surf