Southern blue whiting

For the purposes of stock assessment it is assumed that there are four main spawning areas of southern blue whiting (SBW) assessed independently: the Bounty Platform stock, the Pukaki Rise stock, the Auckland Islands stock and the Campbell Island stock (Hanchet, 1999). The Campbell Island Rise stock comprises two spawning grounds, north and south, which are assessed jointly (MPI, 2013a).

The southern blue whiting stocks of New Zealand are noted for highly variable recruitment; which can result in infrequent very strong year classes, followed by long periods of average or below average recruitment (Ministry for Primary Industries 2017). The drivers of these dynamics are not well understood, and forecasts often have high uncertainty (Ministry for Primary Industries 2017). Available biomass can change abruptly and dramatically with the coming and going of a strong year class, making for challenging management that must be able to respond quickly in order to optimize harvest and sustain yields over periods of high and low abundance(Ministry for Primary Industries 2017).

Stock assessment is performed by the Ministry for Primary Industries’ (MPI’s) Fisheries Assessment Working Group (DWWG); but they do not make management decisions.

An updated assessment was performed for this stock in 2018 (Fisheries New Zealand 2019); it was not updated for 2019. This assessment used a wide-area acoustic survey as a fishery independent data set. Commercial catch and catch at age was used as fishery dependent data. The CASAL (C++ Algorithmic Stock Assessment Laboratory) model framework uses Bayesian statically derived methods and allows for likelihood or posterior profiles. It can also generate Bayesian posterior distributions using Monte Carlo Markov Chain (MCMC) methods and allows for projections with uncertainty. New in this most recent update was the extension of the assessment timeseries back to 1960 where it was only from 1979 forward in past assessments. Also new to this assessment was the use of an informative prior on the acoustic survey catchability. These two improvements resulted in only a slight change in B₀ estimates of ~ 3,000 t (Fisheries New Zealand 2019).

To maintain sustainable yields of this stock, the latest advice is based on conservative abundance estimates. Advised total allowable catches (TACs) are assumed to equal the set TAC, as they are determined by the same entity: the total allowable commercial catch (TACC) was at 29,400 tonnes for the 2011-2012 through 2013-2014 fishing seasons, and since that time has been at 39,200 tonnes (Ministry for Primary Industries 2017).

For the 2018 fishing year, 39,200 mt was advised in the assessment report, but alternative runs and risk profiles were shown at the approximate current catch (23,000 t) and 40,000 t (Fisheries New Zealand 2018). As the assessment advice and management decisions are made by the same government agency, set quota also equals advised quota for this stock. This same advice was carried forward for the 2019 fishing year (Fisheries New Zealand 2019). Projections during the 2018 assessment indicated that at the then current catches for that year the stock would be above 40% of B₀ for the next 5 years (Fisheries New Zealand 2019).

A biomass reference point of 40% of B₀ (Equilibrium “virgin” biomass: the population that would occur in the absence of fishing) was adopted as the management target for the fishery. According to the latest estimates of B₀ (345,000 tonnes), B_lim = B_20% is at 69,000 tonnes (20% o(Fisheries New Zealand 2019) B₀) and B_trp (40% B₀) at 138,000 tonnes (Fisheries New Zealand 2019). Management also defines a Hard limit  at 10% of B_0, a “biomass level below which a stock is deemed to be “collapsed” where fishery closures should be considered in order to rebuild a stock at the fastest possible rate”, and an overfishing threshold at F_40% of B₀ (Ministry for Primary Industries 2017); (Ministry of Fisheries 2008) .

Based on the most recent assessment (Fisheries New Zealand 2018)(Fisheries New Zealand 2019) biomass for this stock is approximately 70% of B₀; well above the B_40% target, B_20% soft limit, and B_10% hard limit. Biomass is >90% likely to be above the target and less that 1% likely to be below hard or soft biomass limits. The stock has been above Btrp since 1994 and has risen since a recent low near B_trp in 2009. A slight downward change in the most recent year is seen in the biomass time series.

Fishing mortality as exploitation (U) is less than 10% and below the target exploitation of 23%. There is a less than 10% chance that the stock is above the exploitation target (). Recruitment is also good, with strong year classes seen in 2007, 2009, and 2011. These represent the second, third and fourth highest year classes in the assessment (Fisheries New Zealand 2018)(Fisheries New Zealand 2019). Since then estimates of recruitment have been below average, but these are considered highly uncertain (Fisheries New Zealand 2018)(Fisheries New Zealand 2019).

Landings have been below quota levels and have fluctuated between 19 and 31,000 t since 2010. 2018 landings were approximately 39% of the quota (Fisheries New Zealand 2019).

Management is conducted by the MPI and the Deepwater Group Limited (DWG Ltd), a non-profit company established to represent quota owners' interests in fisheries science and management (Akroyd et al. 2012).

The New Zealand’s Ministry of Fisheries (NZMF) has managed the fishery under a Quota Management System (QMS) since 1999 in order to maintain stock biomass levels that support a Maximum Constant Yield (MCY) (MPI, 2013a,b).

Managers define three biomass targets or limits used to promote sustainability of this stock a target biomass (B_trp) which is equal to 40% of B₀ (B_40%), a “soft” limit of 20% B0 (B_20%) and a “hard” limit equal to 10% of B0 (B_10%). At B_20% managers are to set quotas in line with rebuilding the biomass back to Btrp, though a timeframe for this is not specified. Should the stock fall below B_10%, a “biomass level below which a stock is deemed to be “collapsed” where fishery closures should be considered in order to rebuild a stock at the fastest possible rate” (Ministry of Fisheries 2008).

Other management measures include a minimum mesh size in trawls at 60 mm, bottom trawl closures to protect sensitive seabed areas, and a ban on use of cables for net sounders (which can entangle some seabirds). The industry also agreed to use sea lion excluder devices (SLEDs) (MPI, 2014c).

Illegal, Unreported and Unregulated (IUU) fishing is considered to be low. The 29,400 tonnes TACC was slightly overpassed in the 2011-2012 season. Official catches since have been well below the 39,200 tonnes TACC (Fisheries New Zealand 2019).  An estimated 40-310,000 tonnes  were illegally discarded in 2004 (Ministry for Primary Industries 2017). Anderson (2009) estimated only 0.23% of the catch to be discarded from 2002-2007; a finding has since been re-affirmed during MSC certification (O’Boyle, R et al. 2018).  The low level attributed to the fact that the fishery primarily targets spawning aggregations (Ministry for Primary Industries 2017). 

Data on interactions with ETP species was last updated in 2018 (Fisheries New Zealand 2019).There are few interactions with ETP species in this fishery. Some interactions have occurred with New Zealand sea lions, New Zealand fur seals, and birds. Data collection and enforcement is high, with near 100% observer coverage and VMS (Vessel Monitoring System) use by all participants.

Observed captures of sea lions in the fishery prompted National Deepwater Plan which includes Operational Objective 2.2: Ensure that incidental New Zealand sea lion mortalities (Fisheries New Zealand 2019). The government instituted a number of requirements including 100% observer coverage and the use of sea lion exclusion devices (SLEDs) on all tows. Since institution in 2014 catches of sea lions have dropped from 21 in 2012-2013 to zero in 2016-2017 (Fisheries New Zealand 2019).

Fur seals, which are considered “Not Threatened” and increasing in abundance (Fisheries New Zealand 2019), have also seen a decline in captures. Total captures and capture rates have declined from 95 captures and a rate of 0.118 per tow in 2013-2014 to 11 and a rate of 0.002 per tow in 2016-2017 (Fisheries New Zealand 2019). This is likely the result of the strategy and mitigation measures implemented for sea lions.

Seabird interactions are another potential area of concern. Salvin's albatross, Grey petrel, and Campbell black-browed albatross are the species most encountered. Of these only Salvin's albatross is listed as threatened by New Zealand (O’Boyle, R et al. 2018). Overall captures have been in decline recently. Captures in 2012-2013 were estimated 19 (0.024 per haul) but declined to 6 (0.011) in 2016-2017 (Fisheries New Zealand 2019). This is in part due to the mandatory use of streamers and other mitigation measures imposed by the national strategy. Overall this fishery has low interactions with sea birds. The fishery accounts for less than 2% of the Salvin's albatross mortality, the highest risk species (O’Boyle, R et al. 2018).

Bycatch data for this fishery was last updated in 2018 (Fisheries New Zealand 2019). Bycatch and retention of other fish species is minimal in this fishery (Fisheries New Zealand 2019)(O’Boyle, R et al. 2018). Less than 1% of the catch is composed on fish species other than blue whiting. These include ling, hake, and hoki, with smaller amounts of porbeagle shark, jack mackerels, rattails, Ray’s bream, and silverside.  Of these only porbeagle sharks is the highest at 0.04% of the catch (O’Boyle, R et al. 2018), and this is a quota monitored stock which is within biologically acceptible levels of removal (Fisheries New Zealand 2019).

Overall a strategy to minimize bycatch is in place (O’Boyle, R et al. 2018) and there is a high degree of observer coverage (near 100%) on fishing trips (O’Boyle, R et al. 2018). Additionally, a Vessel Monitoring System (VMS) is used to track the vessels for enforcement.

Benthic habitat interactions are low for the pelagic or semi pelagic gear used in this fishery. However 55% of tows are at or near the bottom (within 5 m) so some interactions are possible. This fishery accounts for 1% of the tows close to the bottom (Fisheries New Zealand 2019)(O’Boyle, R et al. 2018). Information and enforcement are high, with 100% observer coverage and the use of VMS. Additionally, protected areas and habitat classification are also in place (Fisheries New Zealand 2019)(O’Boyle, R et al. 2018). However direct impacts of the gear on the physical environment have not been undertaken on the fishing grounds. Detailed high quality habitat maps of the area are not available (O’Boyle, R et al. 2018). This is unsurprising as this fishery operates in the deep water (300-600 m). While overall fishery impact is likely low, more research is needed (O’Boyle, R et al. 2018).

Overall a habitat strategy is in place which attempts to monitor and classify the fisheries impact (O’Boyle, R et al. 2018). This includes both monitoring, increased habitat mapping, work on physical impacts, and a network of Marine Protected Areas. Some of this work is still ongoing, however.

Blue whiting are considered mid-trophic level fish, eating mostly small fish and invertebrates while being prey to larger fish, sharks and marine mammals (Fisheries New Zealand 2018)(Fisheries New Zealand 2019). Some juveniles are prone to bird predation. While some trophic and ecosystem modeling has been undertaken, results are now rather dated (O’Boyle, R et al. 2018). Some work indicates that increases in less resilient species have occurred (Fisheries New Zealand 2018)(Fisheries New Zealand 2019), though it is not clear if this is the result of the 2000-2009 decline in blue whiting abundance. It should be noted that this particular ecosystem has many trophic interactions and is quite complex, suggesting that removal of any particular species might not upset the overall complexity of the system (O’Boyle, R et al. 2018). Overall it could be concluded that while data and analysis are limited, the multiple trophic pathway of this ecosystem make major harvest impacts unlikely.

However, there is also limited comprehensive management engagement on this subject. Ecosystem management of potential impacts is based around a well-structured, legislative, policy and operational framework (O’Boyle, R et al. 2018). But only a partial strategy is implemented through a patchwork of regulations, voluntary agreements, and other measures (O’Boyle, R et al. 2018).