We also provide our clients with property per-risk, marine and aerospace reinsurance products. Unlike property catastrophe reinsurance, which protects against the accumulation of a large number of related losses arising out of one catastrophe, per-risk excess of loss reinsurance protects our clients against a large loss arising from a single risk or location. Substantially all of our property per-risk, marine and aerospace business is also written on an excess-of-loss basis with aggregate limits on our exposure to losses.
Since our investment strategy is to invest a significant portion of our investment portfolio in hedge funds and other privately held securities, which are accounted for under the equity method, or in some cases as a trading portfolio, net realized and unrealized gains (losses) on such investments may have a greater effect on our results of operations at the end of any reporting period than would be the case for other insurance and/or reinsurance companies.
Gross written premiums for 2001 increased 7.9% to $290.2 million from $269.0 million for 2000, while net premiums written declined 10.6% to $154.5 million versus $172.7 million for 2000. Excluding the impact of the September 11th terrorist attacks and the Company's London operations, the Company had gross premium written growth in the catastrophe and risk excess segment and net premium written growth in the catastrophe and risk excess and finite segments. Net premiums earned for 2001 increased 1.2% to $162.1 million from $160.2 million for 2000, reflecting the September 11th terrorist attacks and cessation of London operations. Excluding the September 11th terrorist attacks and the London operations, the Company had net premiums earned growth in all segments except our Other Lines segment. Gross premiums written decreased by $31 million, net premiums written decreased by $21.6 million and net premiums earned decreased by $15.7 million in 2001 compared to 2000 as a result of the cessation of our underwriting activities in London.
Net investment income for the year ended December 31, 2000 decreased 36.3% to $30 million from $47.2 million for 1999. The decrease in net investment income was caused primarily by certain alternative investments (which are carried on the equity method, for which the unrealized gains and losses in each case are recorded through the income statement) and trading portfolio, which produced a loss of $3.5 million for 2000 reflecting a loss in two partnerships, compared to income of $11.1 million for 1999. Our pre-tax annualized investment yield was 6.1% for 2000 compared with 10.4% for 1999, both calculated using amortized cost and investment income before investment expenses. Net realized investment gains for 2000 were $3.2 million, compared to losses of $3.8 million for 1999, reflecting the restructuring of the investment portfolio and sale of Transnational Insurance in 2000 and losses from trading of weather contracts in 1999.
Our hedge fund managers invest in a variety of markets utilizing a variety of strategies, generally through the medium of private investment companies or other entities. Criteria for the selection of hedge fund managers include, among other factors, the historical performance and/or recognizable prospects of the particular manager and a substantial personal investment by the manager in the investment program. However, managers without past trading histories or substantial personal investment may also be considered. Generally, our hedge fund managers may be compensated or receive profit participations on terms that may include fixed and/or performance-based fees or profit participations.
Net cash flow provided by operations was $25.1 million in 2001 compared to $24.3 million used by operations in 2000 due to the effects of timing of collection of receivables and reinsurance recoverables and payments of losses.
We believe that reinsurance receivables and payables do not expose us to significant interest rate risk and are excluded from the analysis below.