Northwest Adriatic is an important reproductive area for two shark species. Dogfishes (Squalus spp.) and smoothhounds (Mustelus spp.) have generally dominated shark bycatch in the northern Adriatic Sea (Barausse et al. 2014; Bonanomi et al. 2018). Between 2006 and 2015, a total of 12,585 hauls from pelagic trawl fisheries were recorded in 3,160 monitored fishing trips in the northern Adriatic. In these hauls, 2,169 Spiny Dogfish and 833 Common Smoothhounds were captured (Bonanomi et al. 2018).

Between April and June across the study period (2006–2015), relatively small individuals (~90 cm total length [TL]) of Common Smoothhound were mostly caught at an average depth of 15 m (± 36 m SD) (Bonanomi et al. 2018). Reported size-at-birth for this species is 24–45 cm TL (Saïdi et al. 2008). Based on catches from a variety of fishing gear, pregnant females and juveniles of this species are found in the northern Adriatic (Bonanomi et al. 2018; Riginella et al. 2020).

Between July and September across the study period (2006–2015), small individuals (~70 cm TL) of Spiny Dogfish were incidentally captured at an average depth of 30 m (± 55 m SD). These were sometimes caught as aggregations of 5 to 146 individuals (Bonanomi et al. 2018). Reported size-at-birth for this species is 18–33 cm TL (Ebert et al. 2021).

Using data from these monitored fishing trips, this area was delineated by the spatial identification of aggregations of immature individuals using the following methods (Bonanomi et al. 2018). ‘Nursery areas’ were identified where large bycatch events of immature individuals were recorded (Bonanomi et al. 2018; note that the use of the term ‘nursery areas’ in the study does not match other ecological uses of the term [e.g., Heupel et al. 2007]). For both Common Smoothhound and Spiny Dogfish, size-at-maturity was the threshold to identify immature individuals. Thus, only bycatch events for each haul with at least 50% immature individuals caught were selected. Following this, aggregations with >18 individual Common Smoothhound and >8 individual Spiny Dogfish were selected, since these thresholds represented the 95th quantile of the distribution of catches of immature individuals. For both species, the minimal area containing the selected events was computed and considered as a potential ‘nursery area’ (Bonanomi et al. 2018). Aggregations of immature individuals for both species in this area resulted in a greater mean catch-per-unit-effort (CPUE) compared with other areas of the northern Adriatic Sea.