The long-term impact of oil pollution on the southern local herd of white whales (Delphinapterus leucas)

The long-term impact of oil pollution on the southern local herd of white whales (Delphinapterus leucas)

25 August 2020

Andrianov V.V., Lebedev A.A. , Neverova N.V. , Lisitsyna T.Yu.

 

INTRODUCTION.

 

The southern local herd (LH) of white whales inhabits the southeastern part of the Onega Bay, White Sea during the summer season (Chernetsky et al. 2002). In the summer of 2003, we began to study this group, and in September, there was an accidental fuel oil spill in the southern part of the Onega Bay (Andrianov et al. 2005). The spill of oil products (OP) caused the contamination of the southeast part of the Onega Bay and covered almost the entire area used by the southern LH (Andrianov et al. 2009). During the summer seasons of 2004–2006, 2011–2013, and 2016–2018, we conducted studies of the white whales inhabiting the southeastern Onega Bay and, in parallel, monitored the level of oil hydrocarbon (OHC) pollution in various parts of the area. The observations of the behavior and distribution changes in white whales have shown their evident relationship with a degree of water pollution in the area. The OP spill has had a negative impact on various members of the trophic chain, including some invertebrates and fishes for which we were able to collect the data (Andrianov et al. 2016). In general, the contamination of the habitat and food objects negatively affected the condition of the southern LH of white whales. To date, enough material has been collected on the dynamics of pollution in the area and variations in the condition of the herd to draw some conclusions on the long-term impact on the southern LH.

 

MATERIAL AND METHODS.

 

The study of white whales was carried out in June - July during 2003–2006, 2011–2013, and 2016–2018. The vesselbased and shore-based observations were conducted, according to the standard technique, continuously in the daytime hours with use of field binoculars BPTS5 8×30лю and YUKON 30×50, and also a spotting scope (30×50). We recorded parameters such as the patterns of animals distribution, size and age structure of groups, degree of fidelity of white whales to a certain area, patterns of animals’ behavior, and impact of weather conditions. Age of white whales was determined by the body color: dark animals were defined as calves (newborns); grey - subadults; and white - adults (Kleinenberg et al. 1964; Ognetov 1981).

 

The groups were divided into mixed-age (adults with calves) and male groups (of exclusively adult animals) (ibid.). When processing the results of shore-based observations, we composed a daily diagram of white whales presence, which showed the number and duration of animals present by the age categories in the observed. The data processed this way were used to calculate the following parameters: the observability factor (Kobs), attendance (A, attendance), and average number (Nav). The methodology for calculating these parameters and composing diagrams can be found in our special publication (Andrianov, Lukin 2017).

 

To assess the level of pollution in the southern Onega Bay in July, 2005, 20 water samples from the nearbottom depth and 22 samples of the bottom sediments were collected from the stations arranged into a grid. In 2011–2013, we performed surveys in the coastal waters from Cape Listvenny to Cape Veinavolok to the 10-m isobath. On June 26, 2011 and July 7 and 12, 2012, 11 water samples were collected from the near the bottom. In 2013, the works were conducted only in the western area, where 6 samples of near-bottom water were collected. During the summer season of 2016, water samples were collected in the water area off Cape Glubokiy at a distance of 250 m from the coast, at 20 stations in 4 steps with an interval of 1/4 of a semidiurnal cycle (to measure the hydrocarbons content in them). In the summer of 2017, 7 water samples were taken from the near-bottom depth at the 1/4 diurnal station and 28 water samples from the surface at 2 semidiurnal stations with an interval of 1 hour. In the summer of 2018, we continued water sampling for determination of OHC. The water samples were collected from the surface directly from shore, every 3 h through the day. The hydrocarbon content in the water and bottom sediment samples was analyzed at the laboratory. The OHC content in the samples, collected from a single stations, was measured by the method of infra-red spectrometry (IRS) (Technique..., 1995, 1998).

 

Black oil-sand aggregates (BOSA) were collected near Cape Glubokiy, if found during low tides. To study the condition of the coastal fish populations, fish were caught off Cape Glubokiy from various coastal sites in 2012, 2013, and 2016–2018. Species composition of catches was determined. Freshwater and sea flounder were analyzed, including: size/weight measurements, identification of sex, and otoliths were collected for age determination in the laboratory. Tissues from invertebrates to determine the weight fraction of OHC were sampled after the animals were live-collected and delivered to the laboratory. Total oil products in tissues of aquatic organisms were determined according to NDI 05.17-2009. Residents of the villages Purnema and Lamtsa reported in their interviews that white whales mortality was registered in 2009–2017. Similar information was received from the officers of the State Inspection on Small-Size Vessels (GIMS), EMERCOM of Russia, and residents of Onega Town.

 

RESULTS AND DISCUSSION.

 

The results of the vessel- and shore-based observations in 2003–2006 have shown that in summer the distribution of white whales in the southeastern part of the Onega Bay has a discrete pattern, but overall the white whales inhibiting this area represent a single herd. During this period, the herd numbered up to 150 individuals (we observed 130 white whales in one aggregation only in July 2005) and had 6 breeding sites with the central site near Cape Glubokiy (Andrianov et al. 2009).

 

Each breeding site of the southern LH is an area of coastal waters (about 5–7 km in length), inhabited by a mixedage group of white whales, up to 30 individuals and consisting of several pods, each of which has its own coastal site for calving. Since 2006, the spatial structure of the herd has been changing. The calving site near Cape Glubokiy is becoming mainly a habitat for adult white whales without calves (male groups). White whales have ceased to use the site off the Lamtsa Village as a habitat; at the sites inhabited by separate groups, we observed white whales leaving the shallow waters. The pattern and frequency of attendance of the area off Cape Glubokiy by white whales changed. Before the oil spill (in 2003) and for some time after (through 2005), the water area off Cape Glubokiy was a breeding site used by white whales for calving and nursing newborns, although in 2005 the OHC content in water, through the most of the area inhabited by the herd, remained at a level 10–15 times higher than the maximum allowable concentration (MAC) limit (Andrianov etc. 2016). As can be seen in the graph, the level of attendance by adults with calves in this area during those years reached 50–60 individuals × hour per an observation day. The sampling and analysis of water from the nearbottom layer, collected in the summer of 2011, showed a 2–3-fold decrease in the concentration of hydrocarbons in the water compared to that in 2005. The shore-based observations in 2011 at Cape Glubokiy showed that the attendance of this area by white whales is high, and remaining at the level of 2003 and 2005.

 

However, it was used mostly by adult white whales without calves (males), similarly to the summer season of 2006. This evidently reflects the decrease in the level of attendance of the area by females with calves in 2011 and 2006, compared to that in the previous years. In 2012, the waters of the southeastern Onega Bay was cleaned of oil deposits to a significant degree as a result of a storm, and the OHC content in water over most of the area decreased to the MAC level. White whales returned to the shallow waters here. In addition, on July 2, 2012 we observed the birth of a calf in the shallow waters near the cape. This was the first such observation since 2003. After the birth, the female with her newborn calf were staying near the cape for a long time, and the attendance of the observed area by other females with calves also increased, as shown by the two peaks of calves’ attendance of the area in the diagram for this period. The changes in the habitat use by the white whales had a variety of consequences that negatively influenced the condition of the herd. The white whales lost a number of adaptive skills, and that could be one of the reasons of an increased mortality rate reported in the interviews. During the four years since 2009, six cases of white whale death (2 adults, 3 calves, and 1 immature) were recorded, whereas in previous years dead animals in the area had been rarely found. A reduction in the level of food supply could become another factor of the increase in the white whales mortality rate. According to the interview data, the status of fish resources in the area such as: herring, flounder, white fish, etc., significantly deteriorated in previous several years. One more possible factor, which could cause the increased mortality rate, is an accumulation of OHC in tissues of the organisms. According to the data obtained by us in 2012–2013, the OHC content in tissues of the aquatic organisms collected from the littoral zone off Cape Glubokiy was dozens of times higher than the OHC level in tissues of the organisms from the non-polluted areas, which was 1–3 mg/kg wet weight (Shchekaturina, 1992).

 

Since invertebrates, as well as fish, are the food items in the diet of white whale, it is logical to expect accumulation of OHC in tissues of white whales also. In 2016–2018, the work on monitoring the ecological condition of the polluted area continued. The study showed that in the coastal zone near Cape Glubokiy clean waters prevailed during various periods of the semidiurnal tidal cycle (Andrianov et al. 2017).

 

In general, according to the studies conducted in 2012, 2013, and 2016–2018, the OHC pollution of the considered coastal waters gradually decreased to the MAC level in most of the area during the summer seasons of those years. In connection with that, changes also occurred in the condition of various elements of the trophic chain, including white whales. The variations in the structure of the populations of freshwater and polar flounder showed a slightly more positive trend compared to that observed previously, as is evidenced by the age structure of flounder in the catches. Thus, in 2012 and 2013 the age structure consisted mainly of 3 year-classes aged 2–4 years, but in 2017–2018 the age structure extended to 5–7 year-classes for both species. The shore-based observations near Cape Glubokiy during the summer seasons of 2016 and 2017 showed that the declined condition of LH inhabiting the waters off the cape remained generally at the level of 2013, but some positive trends in behavior of white whales were also noted. Thus, while the attendance of the study area by white whales remained at the lowest level (it was lower only in 2004, immediately after the oil spill), the pattern of this attendance is improving compared to that in the period 2011–2013. White whales began to approach closer to the coast, which may indicate a lack of oil pollution in those coastal waters.

 

Also, white whales used the cyclic tidal current for stay locally in the habitat: they were mostly lying on the water surface and moved in both directions with the tide. There were changes in white whales behavior that were negative for the population: in the summer of 2017 white whales were seen to leave their calves in care of one or two adult white whales, and several times the calves appeared without any adult supervision. This is dangerous for calves in such difficult hydrological conditions that exist in the southeastern part of the Onega Bay, as was subsequently confirmed by an incident. It is possible that, females went to feed in remote waters from the breeding sites, thus abandoning the calves for an extended period of time. The reason for that could be a redistribution of their food objects and a decrease in their stocks in the area. In their interviews, local residents from the Purnema Village reported the continuing high mortality of the white whales which had begun in 2009 and counted 12 individuals, including: 5 adults, 2 subadults, and 5 calves by 2017. In 2017, a dead calf was found in the area off Purnema Village, where the low tide zone reaches more than a kilometer. The calf was apparently left by adult animals and could not leave the shallow zone on its own in time.

 

CONCLUSION.

 

As a result, the long-term negative consequences to the southern local herd of white whales are evident. In June 2004, when the herd arrived in the area of their summer habitat, the water pollution was so high that the animals left the site. But in the summer of 2005 the herd (counting approximately 150 individuals of different ages) appeared again in area, despite the OHC content in water exceeded the MAC level 10–15 times. In all the subsequent years up to 2017 animals used the area of summer habitat. But the years of staying in the polluted waters possibly had an impact on the white whales. The negative effect became apparent since 2009, when dead white whales were first recorded in the area. Although the actual cause of mortality rate increase (that happened in the following years) was possibly the changes in the spatial structure of herd and food, all of which was a side effect of pollution. Currently, the area inhabited by white whales has been mostly cleaned of OHC, but white whales continue to die.

 

There are also other signs of gradual stabilization of the ecosystem (besides improvement in the water quality) which has suffered from pollution: increase in the numbers of fish and invertebrate populations which are the food objects of white whales. If the condition of the ecosystem continue to improve, we can expect that the same would happen to the local white whales’ herd (probably at a new level). This will occur when the death rate of white whales returns to the level recorded before the pollution, when records of dead white whales were very rare.

 

KEY RESULTS:

 

1. The accidental spill of fuel oil in the southern part of the Onega Bay has led to a serious disturbance of the coastal ecosystem inhabited by the southern local herd of white whales.

 

2. During almost 10 years after the spill, the level of pollution of waters in the study area exceeded the maximum allowable concentration (MAC) limit several times. OHC accumulated in tissues of aquatic organisms; the populations of freshwater and polar flounder caught in water area consisted mainly of 3 year-classes: aged 2–4 years.

 

3. During the period from 2009 to 2017, a high mortality of white whales and a decline in the size of the herd was observed. 4. The studies in 2016–2018 showed that the water area was mostly cleanned of OHC. In tissues of aquatic organisms, the content of OHC also decreased to a normal level; the population trends of freshwater and polar flounder has improved. In 2018, no mortality of white whales was observed. 5. When carrying out further studies, we may expect a recovery of the southern local herd of white whales with the restoration of ecological equilibrium to the coastal ecosystem in the southeastern part of the Onega Bay.

 

References

 

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Andrianov V.V., Lebedev A.A., Neverova N.V., Lukin L.R., Vorobyeva T.Ya., Sobko E.I., Kobelev E.A., Lisitsyna T.Yu., Samokhina L.A., Klimov S.I. 2016. Long-term environmental impact of an oil spill in the southern part of Onega Bay, the White Sea. Russian Journal of Marine Biology, 42(3): 205–215.

 

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