ORIGINAL PAPER Inﬂuence of increased cod abundance and temperature on recruitment of northern shrimp (Pandalus borealis) Ingibjo ¨rg G. Jo ´nsdo ´ttir • A ´ rni Magnu ´ sson • Unnur Sku ´ lado ´ttir Received: 4 July 2012 / Accepted: 8 January 2013 / Published online: 22 January 2013 Ó Springer-Verlag Berlin Heidelberg 2013 Abstract Despite an increase in northern shrimp (Panda- lus borealis) female biomass in the past years, the recruit- ment of the offshore population north and northeast of Iceland has remained very low. In this study, the inﬂuence of abiotic and biotic factors was studied in relation to shrimp recruitment. Two factors, cod (Gadus morhua) abundance and summer sea surface temperature (SST), were found to have a negative effect on offshore shrimp recruitment, explaining 71 % of the observed variation. Both cod abun- dance and temperature on the offshore shrimp grounds have increased in the past years, while recruitment has decreased and been at historically low levels since 2005. No signiﬁcant relationship was found between recruitment and spawning biomass, indicating that recruitment variability is mainly driven by other factors. Cod abundance and summer SST are likely to affect different life stages of shrimp, as SST inﬂu- ences shrimp during its planktonic phase while cod abun- dance inﬂuences the demersal stage. Introduction Recruitment is often deﬁned as the number of young-of-the- year surviving and joining a population. Recruitment is a complex process driven by the biological and physical var- iability of the environment (Cushing 1996; Chambers and Trippel 1997). The factors determining the recruitment, however, vary both spatially and temporally, resulting in inconsistent recruitment. The year class strength of many marine species is probably largely determined during the early life stages. Reliable information on recruitment is important for effective management of the exploited stock. However, the population dynamics during the ﬁrst years are often poorly known, and the processes causing the interan- nual variations in recruitment remain largely unexplained. Northern shrimp (Pandalus borealis) has a circumpolar distribution and is widely distributed in the northern hemisphere (Bergstrom 2000). In Icelandic waters, off- shore shrimp is found in the waters north and northeast of Iceland. Offshore shrimp ﬁshing started in the early 1970s and shrimp was one of the most important commercial species in Icelandic waters in the 1980s and 1990s. Shrimp ﬁshing peaked in 1994–1997 when the annual catch reached 65,000 t (Anonymous 2012a). In the following years, shrimp biomass and catch decreased and the biomass reached historically low values in 2004 (Anonymous 2012a). Since 2004, the shrimp biomass has increased slightly but despite very limited ﬁshing in the past 7 years, the shrimp biomass has remained lower than the biomass in the years 1990–1997. In contrast, the female biomass index has increased since 2004, perhaps due to limited ﬁshing, and was above average between 2007 and 2009. The recruitment, which is estimated as 2-year-old shrimp, has been highly variable in the period between 1988 and 2011 but has been well below average since 2004 (Anonymous 2012a). Despite increasing female biomass from 2004 to 2009, recruitment has remained low (Anonymous 2012a). One of the reasons for slow recovery of the shrimp stock may be low recruitment, since no strong year classes have appeared since the 2000 year class. Shrimp recruitment variability is inﬂuenced by various factors, both abiotic and biotic. The most studied abiotic factor is temperature (Wieland 2005; Aschan and Communicated by X. Irigoyen. I. G. Jo ´nsdo ´ttir (&)  A ´ . Magnu ´sson  U. Sku ´lado ´ttir Marine Research Institute, Sku ´lagata 4, P.O. Box 1390, 121 Reykjavı ´k, Iceland e-mail: ingibj@hafro.is 123 Mar Biol (2013) 160:1203–1211 DOI 10.1007/s00227-013-2172-1