Investment group City of London Investment Trust lifted its net asset value in the first half but suffered plunging net returns on ordinary activities before tax from £100.22m to £25.62m.Limited exposure to the oil and mining sectors saw the trust's net asset value per share rise 2.1% in the six months to the end of December on a total return basis.Net asset value per share was 378.5p, slightly up from 377.5p on 30 June, while return per share was down from 37.8p to 8.71p.But gross revenue and capital gains fell sharply, declining from £104.82m - a figure which was boosted by strong capital return - to £30.98m.The figure was above the 1.2% increase in the group's benchmark the AIC UK Equity Income Sector, while the company's performance was also ahead of the 1.6% increase registered in the OEIC sector and the 0.4% gain in the FTSE All Share Index.City of London shares were down 0.16% to 396.80 at 10:17 on Thursday.