Abalone recruitment patterns before and after sea urchin barrens formation in northern California: incorporating climate change

ABSTRACT Understanding the recruitment dynamics of invertebrates in kelp forests is critical to informing climate‐ready restoration. Here we examine abalone and sea urchin recruitment (3–20 mm in size) patterns in northern California across a period of drastic change. Annual surveys were conducted before, during and after the MHW (2014–2016), the loss of a major predatory sea star (2012–2016) and the collapse of a bull kelp forest in 2014. Divers surveyed artificial reef recruitment modules (n = 12) over 20 years in an area that once supported dense bull kelp, Nereocystis leutkeana , forests and the world's largest recreational abalone fishery. From 2016 to 2022, we tracked the decline of red abalone, Haliotis rufescens , recruitment and the rise of purple sea urchin, Strongylocentrotus purpuratus , recruitment. Adult densities of purple sea urchins increased as did newly settled sea urchins (<3 mm), while adult and newly settled red abalone declined. Eight years after the kelp forest collapse, red abalone recruitment remained low and sea urchin recruitment continued to increase. Recruitment patterns can inform both abalone restoration targets and sea urchin dynamics as part of a more holistic kelp forest recovery plan that is responsive to climate change drivers.